If "Preview generation failed" for some images in the Files section, how do I trigger a new attempt. (The images are ok, they render properly in full browser window size.)
first you can try to delete the whole image cache from your browser and make a hard reload.
Otherwise you can connect via finder to your protonetbox and enable “showing of hidden files”. Then you will see a “.protonet” folder. In this place are the thumbnails for preview stored. You can delete this folder and refresh the page in your browser.
This should help.
Good. Deleteing blabla/.protonet solved the issue.