Anki-sync-server: Media does not sync in Anki Desktop but syncs in AnkiDroid

I’ve set up a custom server to handle my decks without relying on Ankiweb, using anki-sync-server and the “custom sync server redirector” addon (358444159)

I thought it worked fine, but after setting up a new computer I realized Anki has synced only the text fields and not the media (images and audio). I used a spare tablet to try a fresh sync to Ankidroid and found that the media files get pulled fine there, so it’s not a case of files being corrupted or misplaced in my server. Seems like a desktop specific issue.

I tried Check Database and Force Sync from the server, but the sync just finishes silently without syncing any media.

When I close Anki after a force sync the Media Sync Log pops up very briefly but disappears before I can copy all of its contents. Is there a way to open it manually?

What error message I could get from the log starts like this:

anki error decoding response body: invalid type: null, expected a string at line 1…

Not sure if this is a desktop Anki problem or an add-on problem. The add-on is dead simple; you just provide a sync address. If I was setting it up wrong it wouldn’t sync the text fields as well. If there was something wrong with the server then AnkiDroid sync would also fail…

Version ⁨2.1.54 (b6a7760c)⁩
Python 3.9.10 Qt 6.3.1 PyQt 6.3.1