Hi all, thanks for responding, I think I found a solution, but it’s not actually what I would like, but it works:
I was able to rollback from PHP 7.3 to PHP 7.2.
It seems there are some compatibility issues with WP 5.3 and PHP 7.3 (to some extend).
I tried to modify server read/write settings; didn’t work
I tried to switch off all plugins: didn’t work
I tried to uninstall other stuff: didn’t work
The only thing that immediately worked: roll back to PHP 7.2
So for me it now works every single time I want to upload an image via PICU Plugin.
Not sure if this is THE solution, but this part now works for me ??