@BrundleFly2077 went through their bot and then a human who advised me to do a (non-destructive) "Repair account" thing from settings, but that didn't help. They also said they're aware of the issue and will update me once it's fixed (but also to provide a reference number for the chat, so does that mean I have to check manually?).
Anyway yeah keep me updated!
@BrundleFly2077 hey! My Kobo synced today, and I later got an email from them saying they've fixed whatever the problem was! Check if it's working for you?
Now to go and re-enable sideload mode, I guess...๐๏ธ