Preview broken

In the last week orso my AfterEffects is completely broken to the point of being unusable (even though I'm still paying a full subscription every month!)

Preview breaks as soon as I make any changes on the timeline.

I open a project, the preview window works fine. I can drag along the timeline and the preview updates.

But as soon as I do anything in the timeline, e.g. move a layer, switch a layer's visibility, literally ANYTHING, the preview doesn't update anymore. So basically it's now an expensive previewing software and I can't make any changes whatsoever.

No system incompatibility issues.

Switched off ALL GPU acceleration.

Cleared caches, moved them to different harddisks.

Purged everything multiple times.

Added more memory allocation.

Restarted rebooted, even reinstalled AE.

Start new project. 

The problem persists. And I really need to use this software for its intended purposes soon!