CSS not working in chrome

Please provide a link to a page on your site where we can see this. Thanks.

https://www.lemonsquid.com/knowledge-base
and
https://www.lemonsquid.com/forums/
mainly these to that highly depend on the custom css

the weird thing is that it sometimes works and then it just stops working

The KB page looks fine to me, but I see a a couploe of errors on some CSS files:

 Failed to load resource: the server responded with a status of 400 ()
/A.,qcustom-css=16a73f5216.pagespeed.cf.s-hA_VlvLf.css:1 Failed to load resource: the server responded with a status of 404 ()
api.js:1 Failed to load resource: the server responded with a status of 400 ()

A.,qcustom-css=16a73f5216.pagespeed.cf.s-hA_VlvLf.css:1 Failed to load resource: the server responded with a status of 404 ()

As these are pagespeed cached files, try clearing the pagespeed cache.

Where can I find this file and is this something I can fix myself?

Check with your host, as pagespeed usually operates at the server level. Do you have any caching plugins? You might try clearing their caches, too.

For as far I know I have no caching plugins, I run Ultimate Member with all the extensions, bbPress and mycred.. but thats basically all I can think off?

Contact your host

Will do!