C1 V21 MacOS thumbs/previews randomly disappear ...

Hello, need some help or hints please.

When I open a catalog in C1/21 all thumbnails are just empty frames. If I click on one the image appears in the viewer. But no preview or thumbnail  is created.

This is a new behavior since I had all previews/thumbs before!

I deleted the cache in one backup catalog - but again, C1 is not creating new previews or thumbs.

The strange thing is I have another catalog which shows thumbs/previews when I open it.
But next time I open they are gone. At another time they come back when scrolling thru.

I double checked with Luminar 4 that my Mac is able to open all my Leica DNGs. All thumbnails are there based on the same image folder that I use for C1.

The drive is checked for permissions Ok. 

How can I force C1 to recreate ALL thumbs and previews again? (about 12000 are missing though)

Or how can I reconnect to the cache files if connection is lost (if that's the case)

Any ideas?

TIA, Uli