[Solved] CSS classes has been disappeared
For people who may have the same problem: THE CAUSE IS IN THE PLUGIN. The error is caused by the tab Settings > General Settings > Minify JS and CSS files to optimize page loading speed.
If you disable it, WpDiscuz crashes.
Please, next time instead of dismissing problems with the plugin and blaming the user it would be good to investigate a bit further.
Regards and thanks for the work anyway.