Lightroom Classic does not make jpg anymore

I have had a lot of troubles since i bought a new iMac 24 with only 8 GB RAM. It just doesn't work with Lightroom. 

My seven year old MacBook Pro is faster and more reliable than the new iMac. 

The latest issue appeared a while ago when Lightroom stopped producing proper jpgs from my RAWfiles.

Lightroom make kind of a preview, but the file has no size. It is 0 MB and unreadable. 

Yesterday I installed the latest version of Lightroom. It didn't help at all.

I have checked all the jpg-settings in the export menu. They seems to be correct. 

Any suggestions for what to do