Thread:Project:Support desk/MediaWiki Update 1.15.3 - 1.20.3: Update script causes error

Hello everyone.

While updating my Wiki from version 1.15.3 to 1.20.3 the update script gives this error: Adding index ls_field_val to table log_search ... An error occurred: Es gab einen Syntaxfehler in der Datenbankabfrage. Die letzte Datenbankabfrage lautete: „ALTER TABLE `log_search` DROP PRIMARY KEY, ADD UNIQUE INDEX ls_field_val (ls_field,ls_value,ls_log_id) “ aus der Funktion „DatabaseBase::sourceFile( /wiki/httpdocs/maintenance/archives/patch-log_search-rename-index.sql )“. Die Datenbank meldete den Fehler: „1091: Can't DROP 'PRIMARY'; check that column/key exists (localhost)“. Backtrace: PHP 5.4.11, MySQL 5.4.11 (cgi-fcgi).
 * 1) 0 /wiki/httpdocs/includes/db/Database.php(916): DatabaseBase->reportQueryError('Can't DROP 'PRI...', 1091, 'ALTER TABLE `lo...', 'DatabaseBase::s...', false)
 * 2) 1 /wiki/httpdocs/includes/db/Database.php(3236): DatabaseBase->query('ALTER TABLE `lo...', 'DatabaseBase::s...')
 * 3) 2 /wiki/httpdocs/includes/db/Database.php(3148): DatabaseBase->sourceStream(Resource id #260, false, false, 'DatabaseBase::s...')
 * 4) 3 /wiki/httpdocs/includes/installer/DatabaseUpdater.php(478): DatabaseBase->sourceFile('/srv/www/vhosts...')
 * 5) 4 /wiki/httpdocs/includes/installer/DatabaseUpdater.php(524): DatabaseUpdater->applyPatch('patch-log_searc...', false, 'Adding index ls...')
 * 6) 5 [internal function]: DatabaseUpdater->addIndex('log_search', 'ls_field_val', 'patch-log_searc...')
 * 7) 6 /wiki/httpdocs/includes/installer/DatabaseUpdater.php(332): call_user_func_array(Array, Array)
 * 8) 7 /wiki/httpdocs/includes/installer/DatabaseUpdater.php(294): DatabaseUpdater->runUpdates(Array, false)
 * 9) 8 /wiki/httpdocs/includes/installer/DatabaseInstaller.php(274): DatabaseUpdater->doUpdates
 * 10) 9 /wiki/httpdocs/includes/installer/WebInstallerPage.php(541): DatabaseInstaller->doUpgrade
 * 11) 10 /wiki/httpdocs/includes/installer/WebInstaller.php(269): WebInstaller_Upgrade->execute
 * 12) 11 /wiki/httpdocs/mw-config/index.php(50): WebInstaller->execute(Array)
 * 13) 12 /wiki/httpdocs/mw-config/index.php(18): wfInstallerMain
 * 14) 13 {main}

What can I do?