Here is the answer:
viewtopic.php?f=6&t=10338Apparently a broken database causes the error. Unfortunately the answer in the forums about "broken database" have no mention of the error codes I was getting (HTTP 503, etc.) so I would have never found the answer in these forums (I actually found this linked in another forum about 503 errors).
Here are the steps in case the other post ever goes away:
Before upgrading, always backup your subsonic folder.
If the database breaks, you can recover some settings :
Users (with their password) and their settings:
They are kept in the subsonic.script file. (steps 1 to 3 are only needed if you didn't remove the subsonic folder before reinstalling)
1. stop subsonic service
2. rename the subsonic/db directory
3. start subsonic service : a new database is created.
4. open the old subsonic.script file (in the directory you renamed)
5. select the lines you want to keep (INSERT INTO MUSIC_FOLDER VALUES... INSERT INTO USER VALUES... INSERT INTO USER_ROLE VALUES ... INSERT INTO USER_SETTINGS VALUES ...)
6. in your subsonic webpage, replace "index.view" with "db.view"
7. paste the selected lines
Now you should have your old user settings