Thanks for coming back to this. The configuration on Windows is installed in %APPDATA%\MusicBrainz\ . You can just enter this in the file Explorer path and it should jump to the proper location depending on where %APPDATA% points to.

But to manually navigate there this is usually placed at C:\UsersYourUserName\AppData\Roaming\MusicBrainz

Move any files there elsewhere or just rename the MusicBrainz folder to something else. If you know start Picard it should recreate the folder with a fresh config.

I'd still be interested if you could share me the config. To avoid personal data (MB username and login token) getting leaked here you can also send it per e-mail to outsidecontext AT metabrainz DOT org. You can also open this file in any text editor and remove the access tokens (look for lines starting with "oauth_" and remove them).

Regarding a pre-release we have still not yet released the 2.9 beta unfortunately. If you want and if you have a Github account you can get a recent development build from https://github.com/metabrainz/picard/actions/runs/4971643177 . But the downloads are only visible if you are logged in to Github.

Sorry I did not see your question before Today when I fully uninstalled the previous version (I cannot check what version that was now as all data was erased (Windows 10 uninstaller says)

I just tried installing 2.8.5 (the latest stable version suggested from MusicBrainz.org). I assume this a fresh install so no *.ini from previous installs are supposed to be on this system. Please, correct me if I am wrong by pointing where I can find the file you need for debugging.
Right after the install succeded, I ran (default option of the installer) Picard, and again the same error:

picard.config.ConfigUpgradeError: Error during config upgrade from version 2.2.0.dev3 to 2.4.0.beta3 using upgrade_to_v2_4_0_beta_3():

Traceback (most recent call last):

File "config.py", line 345, in run_upgrade_hooks

File "config_upgrade.py", line 309, in upgrade_to_v2_4_0_beta_3

AttributeError: 'list' object has no attribute 'split'

So there seems to be some remanents of a previous 2.4.0.beta3 install as you proposed.
Could please point me to where that file can be?
or maybe a link to the newest version patched to overcome this issue?
I just see the v. 2.8.5 available to download.

Thanks.

Sorry I did not see your question before Today when I fully uninstalled the previous version (I cannot check what version that was now as all data was erased (Windows 10 uninstaller says)

I just tried installing 2.8.5 (the latest stable version suggested from MusicBrainz.org). I assume this a fresh install so no *.ini from previous installs are supposed to be on this system. Please, correct me if I am wrong by pointing where I can find the file you need for debugging.
Right after the install succeded, I ran (default option of the installer) Picard, and again the same error: 

See code changes in pull request #2203 submitted by phw.

Thanks for the report. From what version did you upgrade? Did you maybe have a development version of Picard 2.4 running?

This looks a bit like you config was a partial upgrade. Do you still have the original Picard.ini from before the upgrade to share?