Error on loading the backend

[MoniqueHahnefeld]

The error does nothing bad, but have to check this...

Warning: mysqli::real_escape_string(): Couldn't fetch mysqli in system/modules/core/library/Contao/Database/Mysqli/Statement.php on line 51

0 [internal function]: __error(2, 'mysqli::real_es...', '/var/www/virtua...', 51, Array)

1 system/modules/core/library/Contao/Database/Mysqli/Statement.php(51): mysqli->real_escape_string('1')

2 system/modules/core/library/Contao/Database/Statement.php(350): Contao\Database\Mysqli\Statement->string_escape('1')

3 system/modules/core/library/Contao/Database/Statement.php(325): Contao\Database\Statement->escapeParams(Array)

4 system/modules/core/library/Contao/Database/Statement.php(262): Contao\Database\Statement->replaceWildcards(Array)

5 system/modules/core/classes/BackendUser.php(162): Contao\Database\Statement->execute('a:12:{s:7:"refe...', '1')

6 [internal function]: Contao\BackendUser->__destruct()

7 {main}

Warning: mysqli::query(): Couldn't fetch mysqli in system/modules/core/library/Contao/Database/Mysqli/Statement.php on line 81

0 [internal function]: __error(2, 'mysqli::query()...', '/var/www/virtua...', 81, Array)

1 system/modules/core/library/Contao/Database/Mysqli/Statement.php(81): mysqli->query('UPDATE tl_user ...')

2 system/modules/core/library/Contao/Database/Statement.php(293): Contao\Database\Mysqli\Statement->execute_query()

3 system/modules/core/library/Contao/Database/Statement.php(264): Contao\Database\Statement->query()

4 system/modules/core/classes/BackendUser.php(162): Contao\Database\Statement->execute('a:12:{s:7:"refe...', '1')

5 [internal function]: Contao\BackendUser->__destruct()

6 {main}

Warning: Contao\Database\Mysqli\Statement::get_error(): Couldn't fetch mysqli in system/modules/core/library/Contao/Database/Mysqli/Statement.php on line 92

0 system/modules/core/library/Contao/Database/Mysqli/Statement.php(92): __error(2, 'Contao\Database...', '/var/www/virtua...', 92, Array)

1 system/modules/core/library/Contao/Database/Statement.php(116): Contao\Database\Mysqli\Statement->get_error()

2 system/modules/core/library/Contao/Database/Statement.php(295): Contao\Database\Statement->__get('error')

3 system/modules/core/library/Contao/Database/Statement.php(264): Contao\Database\Statement->query()

4 system/modules/core/classes/BackendUser.php(162): Contao\Database\Statement->execute('a:12:{s:7:"refe...', '1')

5 [internal function]: Contao\BackendUser->__destruct()

6 {main}

Fatal error: Uncaught exception 'Exception' with message 'Query error: (UPDATE tl_user SET session='' WHERE id='')' in /xxxpfadxxx/system/modules/core/library/Contao/Database/Statement.php:295 Stack trace: #0 /xxxpfadxxx/system/modules/core/library/Contao/Database/Statement.php(264): Contao\Database\Statement->query() #1 /xxxpfadxxx/system/modules/core/classes/BackendUser.php(162): Contao\Database\Statement->execute('a:12:{s:7:"refe...', '1') #2 [internal function]: Contao\BackendUser->__destruct() #3 {main} thrown in /xxxpfadxxx/system/modules/core/library/Contao/Database/Statement.php on line 295