Resolved PHP Fatal error: Uncaught exception 'RuntimeExcept

We are trying to import categories (in the past it was working), but now we are receiving error. In admin there is error 500 server error. At Apache log, we found this:

[Wed Jul 06 12:32:19.085508 2016] [:error] [pid 8266] [client 78.102.83.58:63707] PHP Fatal error: Uncaught exception 'RuntimeException' with message 'Could not connect to MySQL.' in /home/test/www/libraries/joomla/database/driver/mysqli.php:183\nStack trace:\n#0 /home/test/www/libraries/joomla/database/driver.php(1726): JDatabaseDriverMysqli->connect()\n#1 /home/test/www/administrator/components/com_csvi/helper/settings.php(45): JDatabaseDriver->loadResult()\n#2 /home/test/www/administrator/components/com_csvi/rantai/model.php(159): CsviHelperSettings->__construct(Object(JDatabaseDriverMysqli))\n#3 /home/test/www/administrator/components/com_csvi/rantai/rantai.php(94): RantaiModel->__construct(Object(JInput))\n#4 /home/test/www/libraries/joomla/application/web.php(260): Import->doExecute()\n#5 /home/test/www/administrator/components/com_csvi/rantai/rantai.php(167): JApplicationWeb->execute()\n#6 {main}\n thrown in /home/test/www/libraries/joomla/database/driver/mysqli.php on line 183, referer: http://dev2.test.cz/administrator/index.php?option=com_csvi&view=import

Can you please help? We don't know what the problem is. We tried to restart MySQL server, but no help.

Thank you