rare 403 on Joomla admin login

I have two news sites running on Litespeed + Cloudflare , running just fine for over a year now and performance is greater then ever! Only one little problem, very rarely my client isn't able to login to joomla admin, it shows the litespeed 403 page and just a few minutes later everything is fine again. Annoyingly the browser often caches the 403 redirect and so he has to use different browser or flush dns / clear cache or even has to restart his pc to get back into the admin panel. Very frustrating if you need that news-item to be posted before any other news-site.

I'm able to get the same error from my location, only when he points it out to me. He also has a colleague who manages the other news site on the same server, they experience the same problem at the same time.

Anyway, I searched a lot and read about permission issues etc. but they seem to be okay since most of the time there is no problem at all... Many issues revolve around permanent 403 errors but none seem to be about occasional 403's. Could this be Cloudflare related? Anyone who has similar experiences?

403 errors happened so rarely that I did not really bother with it, but they seem to get worse now for whatever reason.