Well, embarrassing for me, but perhaps good news for you is that there was nothing wrong with Collabora after all. It was my browser blocking access to the Zotero API. I enabled the debug menu as instructed (maybe unnnecessary?), opened the Developer console, and there saw a line saying it couldn’t access the key. Changed the settings, now it’s working. I needed the help because it wouldn’t occur to me that it wasn’t a Collabora issue but a browser issue, so I thank you for the kind help. Cheers!