GPG error on updating after upgrade to Bullseye

After upgrading from Buster to Bullseye, I was having issues with Transmission, and I was going to reinstall it, but now I’m getting the following error:

Reading package lists... Error!
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://archive.raspberrypi.org/debian bullseye InRelease: Splitting up /var/lib/apt/lists/archive.raspberrypi.org_debian_dists_bullseye_InRelease into data and signature failed
W: Failed to fetch https://archive.raspberrypi.org/debian/dists/bullseye/InRelease  Splitting up /var/lib/apt/lists/archive.raspberrypi.org_debian_dists_bullseye_InRelease into data and signature failed
W: Some index files failed to download. They have been ignored, or old ones used instead.
E: Splitting of clearsigned file /var/lib/apt/lists/archive.raspberrypi.org_debian_dists_bullseye_InRelease failed as it doesn't contain all expected parts
E: The package lists or status file could not be parsed or opened.

Now, I know that, because I’m still using Buster repositories for Mono and Sonarr, I must have buggered something up in the process of the upgrade, but since I’m still recovering from an infection with a certain real world virus that’s been messing up societies all over the globe, I’m drawing a blank on what I messed up.

Clean reinstalling will not be an option for the next week, so any suggestions are welcome.

can you share full output of

apt update

Thanks for the attempt, but I solved the issue in the meanwhile, had a colleague look at it who redid the upgrade process from Buster to Bullseye, and that fixed the issue (he said I probably forgot a step between the update and editing the list.d items for Sonarr and Mono, he just rolled back to buster and then redid the process).

yeah Sonarr and Mono don’t support Bullseye yet. For these you need to stay at Buster as described on our Upgrade Blog.

Correct, and I did, but somewhere along the way I did something wrong.
I know that when I initially followed the guide I edited the list.d items for Mono and Sonarr, my guess is I either made a typo somewhere or forgot to edit something.
Not a clue what at this point, but hey, it works again, and a lesson was learned; don’t upgrade a Linux distro when you’re still slightly feverish from a Covid infection, 'cause the likelyhood of typos and other mistakes is high :cry:

Who knows. Good it was solved and upgrade completed.