Deinstalling the template is not possible since this would remove all the links from the menu entries to the template styles. But I did install the latest version of the template. Now I do not see any difference in file size on the different files from my web site to the install package. I also see that the latest modification date changed to today. But still whenever I add a (even empty) custom.css file the whole SP page builder breaks. Can you tell me which modification you identified as the route cause?
When I enable System debugging I get this Log message:
assets - No asset found for com_sppagebuilder.page.3, falling back to com_sppagebuilder
Is this maybe linked to the problem?