@BrundleFly2077 okay, so one step ahead. I had my account on sideload mode to start with. After signing into my Kobo account again, I was able to sign in to Pocket as well.
But I'm still having sync issues; it's back to the same "Sync failed" followed by "Something went wrong and we couldn't completely sync your eReader. Please try again" while my Pocket list remains empty ๐
I wrote to Mozilla about it and linked to this thread (hi, Mozilla ๐) so let's see what happens. I'm thinking of writing to Kobo too
@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!