@malprik - Please enable debug like shown at https://kb.mailwizz.com/articles/enable-debugging/ then refresh the page. You should get the reason for the error.
One hint is that the login page has a big image on it which has to be resized. Some PHP hosts will have a very low memory limit, like 32MB/RAM, which is super small, so the solution is to increase the memory limit to something around 128MB/RAM, restart the web server and the issue should go away.