White page in browser

Today suddenly i got white page instead of web-gui.

I checked logs and got in error_log something like that:

[Fri Jul 09 14:05:21.268335 2021] [php7:error] [pid 2497] [client 172.22.27.132:17285] PHP Fatal error: Uncaught Error: Call to undefined function phpFastCache\\Core\\Pool\\IO
mb_strtolower() in /opt/observium/libs/phpFastCache/Core/Pool/IO/IOHelperTrait.php:55\nStack trace:\n#0 /opt/observium/libs/phpFastCache/Drivers/Sqlite/Driver.php(92): phpFastCache\\Drivers\\Sqlite\\Driver->getPath()\n#1 /opt/observium/libs/phpFastCache/Drivers/Sqlite/Driver.php(100): phpFastCache\\Drivers\\Sqlite\\Driver->getSqliteDir()\n#2 /opt/observium/libs/phpFastCache/Drivers/Sqlite/Driver.php(75): phpFastCache\\Drivers\\Sqlite\\Driver->driverCheck()\n#3 /opt/observium/libs/phpFastCache/CacheManager.php(218): phpFastCache\\Drivers\\Sqlite\\Driver->__construct(Array)\n#4 /opt/observium/includes/cache.inc.php(548): phpFastCache\\CacheManager::getInstance('Sqlite')\n#5 /opt/observium/html/includes/functions.inc.php(18): include_once('/opt/observium/...')\n#6 /opt/observium/html/index.php(15): include_once('/opt/observium/...')\n#7 {main}\n thrown in /opt/observium/libs/phpFastCache/Core/Pool/IO/IOHelperTrait.php on line 55

I didn't make any changes it just happened.