I've investegated further and have got some log-entries, which might help to inditify the problem at hand.
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[23-Nov-2011 15:00:21] PHP Notice: Illegal member variable name in C:\inetpub\wwwroot\lib\dml\sqlsrv_native_moodle_recordset.php on line 0
[23-Nov-2011 15:00:21] PHP Stack trace:
[23-Nov-2011 15:00:21] PHP 1. {main}() C:\inetpub\wwwroot\admin\index.php:0
[23-Nov-2011 15:00:21] PHP 2. upgrade_core($version = 2011033005.08, $verbose = TRUE) C:\inetpub\wwwroot\admin\index.php:273
[23-Nov-2011 15:00:21] PHP 3. xmldb_main_upgrade($oldversion = '2008073113') C:\inetpub\wwwroot\lib\upgradelib.php:1384
[23-Nov-2011 15:00:21] PHP 4. upgrade_migrate_files_blog() C:\inetpub\wwwroot\lib\db\upgrade.php:600
[23-Nov-2011 15:00:21] PHP 5. moodle_database->count_records_select($table = 'post', $select = 'module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = *uninitialized*, $countitem = *uninitialized*) C:\inetpub\wwwroot\lib\db\upgradelib.php:405
[23-Nov-2011 15:00:21] PHP 6. moodle_database->count_records_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1499
[23-Nov-2011 15:00:21] PHP 7. moodle_database->get_field_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $strictness = *uninitialized*) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1516
[23-Nov-2011 15:00:21] PHP 8. moodle_database->get_record_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $strictness = 0) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1345
[23-Nov-2011 15:00:21] PHP 9. sqlsrv_native_moodle_database->get_records_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $limitfrom = 0, $limitnum = 0) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1270
[23-Nov-2011 15:00:21] PHP 10. sqlsrv_native_moodle_recordset->current() C:\inetpub\wwwroot\lib\dml\moodle_database.php:811
[23-Nov-2011 15:00:21] PHP Notice: Illegal member variable name in C:\inetpub\wwwroot\lib\dml\sqlsrv_native_moodle_database.php on line 812
[23-Nov-2011 15:00:21] PHP Stack trace:
[23-Nov-2011 15:00:21] PHP 1. {main}() C:\inetpub\wwwroot\admin\index.php:0
[23-Nov-2011 15:00:21] PHP 2. upgrade_core($version = 2011033005.08, $verbose = TRUE) C:\inetpub\wwwroot\admin\index.php:273
[23-Nov-2011 15:00:21] PHP 3. xmldb_main_upgrade($oldversion = '2008073113') C:\inetpub\wwwroot\lib\upgradelib.php:1384
[23-Nov-2011 15:00:21] PHP 4. upgrade_migrate_files_blog() C:\inetpub\wwwroot\lib\db\upgrade.php:600
[23-Nov-2011 15:00:21] PHP 5. moodle_database->count_records_select($table = 'post', $select = 'module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = *uninitialized*, $countitem = *uninitialized*) C:\inetpub\wwwroot\lib\db\upgradelib.php:405
[23-Nov-2011 15:00:21] PHP 6. moodle_database->count_records_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1499
[23-Nov-2011 15:00:21] PHP 7. moodle_database->get_field_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $strictness = *uninitialized*) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1516
[23-Nov-2011 15:00:21] PHP 8. moodle_database->get_record_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $strictness = 0) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1345
[23-Nov-2011 15:00:21] PHP 9. sqlsrv_native_moodle_database->get_records_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $limitfrom = 0, $limitnum = 0) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1270
[23-Nov-2011 15:00:21] PHP 10. reset(...) C:\inetpub\wwwroot\lib\dml\sqlsrv_native_moodle_database.php:812
[23-Nov-2011 15:00:21] PHP Warning: Cannot use a scalar value as an array in C:\inetpub\wwwroot\lib\dml\sqlsrv_native_moodle_database.php on line 818
[23-Nov-2011 15:00:21] PHP Stack trace:
[23-Nov-2011 15:00:21] PHP 1. {main}() C:\inetpub\wwwroot\admin\index.php:0
[23-Nov-2011 15:00:21] PHP 2. upgrade_core($version = 2011033005.08, $verbose = TRUE) C:\inetpub\wwwroot\admin\index.php:273
[23-Nov-2011 15:00:21] PHP 3. xmldb_main_upgrade($oldversion = '2008073113') C:\inetpub\wwwroot\lib\upgradelib.php:1384
[23-Nov-2011 15:00:21] PHP 4. upgrade_migrate_files_blog() C:\inetpub\wwwroot\lib\db\upgrade.php:600
[23-Nov-2011 15:00:21] PHP 5. moodle_database->count_records_select($table = 'post', $select = 'module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = *uninitialized*, $countitem = *uninitialized*) C:\inetpub\wwwroot\lib\db\upgradelib.php:405
[23-Nov-2011 15:00:21] PHP 6. moodle_database->count_records_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1499
[23-Nov-2011 15:00:21] PHP 7. moodle_database->get_field_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $strictness = *uninitialized*) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1516
[23-Nov-2011 15:00:21] PHP 8. moodle_database->get_record_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $strictness = 0) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1345
[23-Nov-2011 15:00:21] PHP 9. sqlsrv_native_moodle_database->get_records_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $limitfrom = 0, $limitnum = 0) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1270
[23-Nov-2011 15:00:21] PHP Fatal error: Call to a member function close() on a non-object in C:\inetpub\wwwroot\lib\dml\sqlsrv_native_moodle_database.php on line 820
[23-Nov-2011 15:00:21] PHP Stack trace:
[23-Nov-2011 15:00:21] PHP 1. {main}() C:\inetpub\wwwroot\admin\index.php:0
[23-Nov-2011 15:00:21] PHP 2. upgrade_core($version = 2011033005.08, $verbose = TRUE) C:\inetpub\wwwroot\admin\index.php:273
[23-Nov-2011 15:00:21] PHP 3. xmldb_main_upgrade($oldversion = '2008073113') C:\inetpub\wwwroot\lib\upgradelib.php:1384
[23-Nov-2011 15:00:21] PHP 4. upgrade_migrate_files_blog() C:\inetpub\wwwroot\lib\db\upgrade.php:600
[23-Nov-2011 15:00:21] PHP 5. moodle_database->count_records_select($table = 'post', $select = 'module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = *uninitialized*, $countitem = *uninitialized*) C:\inetpub\wwwroot\lib\db\upgradelib.php:405
[23-Nov-2011 15:00:21] PHP 6. moodle_database->count_records_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1499
[23-Nov-2011 15:00:21] PHP 7. moodle_database->get_field_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $strictness = *uninitialized*) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1516
[23-Nov-2011 15:00:21] PHP 8. moodle_database->get_record_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $strictness = 0) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1345
[23-Nov-2011 15:00:21] PHP 9. sqlsrv_native_moodle_database->get_records_sql($sql = 'SELECT COUNT(\'x\') FROM {post} WHERE module=\'blog\' AND attachment IS NOT NULL AND attachment <> \'1\'', $params = NULL, $limitfrom = 0, $limitnum = 0) C:\inetpub\wwwroot\lib\dml\moodle_database.php:1270
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