Keijo,
check first that your standard theme is from same version (same package) of moodle as the original upgraded moodle. Copy also all lib files from the original upgraded package (ftp may sometimes cause problems...). Finally check that site config.php and standard theme config.php have no empty lines or white spaces at the end or beginning (after last ?> or before first <?php )
This kind of problems come usually from some edited or missing file...