Try the simple thing first!

Go to the forum admin > toolbox > housekeeping and run both of the tasks in the section ‘Reset combined CSS/JS‘. Let’s see if that fixes it first.

No luck. I reseted all the caches, even the wordpres one

That’s a shame.

Our themes use a php file for the styling. This is loaded as if it were a CSS file and is not an uncommon practice. It is rare we encounter this but the odd hosting operator appears to disallow using a php file as a css file. Actually, I personally have no idea how they do this but it appears to be the case.

I think the next step is to ask your hosting support if they are blocking this and if so request that ot be unblocked. Worth a try…

By the way – when testing to see if it works properly you should also turn off the CSS cache at the forum admin > options > global settings. When things are normal it is then worth turning it on again.

The hosting just told me they don’t have any kind of restrictions. Could it be something in my htaccess file causing this?

That is entirely possible but I am afraid this is an area I have no expertise in myself. My colleague @mr-papa might have an ideas on that but he will not be around until much later in the day.

I can say that a normal WordPress dictated htaccess is fine so it would only come down to added custom entries. Maybe you could take a look and see if anything looks possible… we know some plugins do add extra stuff in there.

I know, I have a lot of things in there… Maybe MrPapa can have a look later

Anything else I can try?

Nothing I know of personally but I am trying to find a topic from about a year ago where someone had a different reason for this. So far not been able to find it but I will keep looking…

Have not yet found the specific one I was looking for but here are links to a couple of solutions – the first is an htaccess issue and the second a problem with Succuri (if you use it)…

Worth a look perhaps:

https://simple-press.com/support-forum/sp5-general-topics/upgraded-to-5-7-but-forum-not-displaying-properly/page-3/

https://simple-press.com/support-forum/sp5-general-topics/entire-forum-display-corrupted-very-strange/page-2/#p161088

Found it, there was an htaccess inside the wp-content with this in it

Order deny,allow
Deny from all
<Files ~ ".(gif|png|jpg|doc|xls|pdf|html|htm|xlsx|docx|mp4|mov|js|css)$">
Allow from all
</Files>

I've renamed it and it works now. 
Should I delete it or edit the content?

There you go. The same scenario as the first link I posted above (post #10).

Excellent.

It will have to be your decision on what to do with it. The question really is – what put it there. As @mr-papa said in the linked thread – WordPress doesn’t put an .htaccess file there so it had to be a deliberate act by someone or something.