The directories are writable, they have the exact same settings/permissions as on the other domain where the AddOns installed just fine. I believe the issue was caused because I had WP set to be using a non-standard (default) directory for media, so in settings I set it back to the default. In a few hours, when traffic dies out a little later, I’ll be ready to try again. I had not added media there for a long time, it is mostly informational.
I’ll drop back by later to tell you how it goes, but I expect it to be fine now. I can see all the logs, it is a vps server and I only bother the host for major things I can’t fix.