This sounds a bit like an error we got some time ago, have a look at http://moodle.org/mod/forum/discuss.php?d=46400#285857
or
http://moodle.org/mod/forum/discuss.php?d=62076#p285859
could that be the problem?
Testing and QA
Error Moodle 1.7 with MSSQL 2005
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