strangely QGIS 3.16, gives the same error, but is able to start

Gtk-Message: 10:50:29.839: Failed to load module "appmenu-gtk-module" no layer

Also can't run in Kubuntu 20.04. In a laptop, the splashscreen does not show, and the fans ramp up to the max speed.

Same on Ubuntu 20.04.2 LTS. Process using 100% CPU, no splash screen...

EDIT
Once the existing local profile from ~/.local/share/QGIS is removed, QGIS starts and works...

Same on Ubuntu 20.04.2 LTS. Process using 100% CPU, no splash screen...

EDIT
Once the existing local profile from ~/.local/share/QGIS is removed, QGIS starts and works...

Yep...it works, thanks for pointing in the right direction.

I managed to get it to work, just by removing the symbology-style.db on the profile folder.

When installed with OSGeo4W installer it also does not run on Windows 10. There is no splashscreen but the process can be seen in the TaskManager.

When installed with OSGeo4W installer it also does not run on Windows 10. There is no splashscreen but the process can be seen in the TaskManager.

No problems here (OSGeo4W v2, installed 3.20.1-1 as update to 3.20.0-5) so whatever the unfortunate problem is, it's not universal.

When installed with OSGeo4W installer it also does not run on Windows 10. There is no splashscreen but the process can be seen in the TaskManager.

I have the same issue in Windows10. I tried to reinstall the previous version but the issue is not solved. The only way to start QGIS was to modify the profiles.ini changing the default profile. In order to load the previous profile, I had to delete the symbology-style.db

The same issue happens in Windows 10, and it seems it only occurs when the default profile has custom saved symbology.

The same issue happens in Windows 10, and it seems it only occurs when the default profile has custom saved symbology.

That seems to be the problem on Windows 10. When I removed symbology-style.db like the Linux folks have done, it solved the problem.

I should mention that the QGIS 3.16.9 update that was done at the same time does not have this problem.

Same on Ubuntu 20.04.2 LTS. Process using 100% CPU, no splash screen...
EDIT
Once the existing local profile from ~/.local/share/QGIS is removed, QGIS starts and works...

Yep...it works, thanks for pointing in the right direction.

I managed to get it to work, just by removing the symbology-style.db on the profile folder.

Same problem: I had to remove all *.db files, but then it worked as usual.

Same on Ubuntu 20.04.2 LTS. Process using 100% CPU, no splash screen...
EDIT
Once the existing local profile from ~/.local/share/QGIS is removed, QGIS starts and works...

Yep...it works, thanks for pointing in the right direction.

I managed to get it to work, just by removing the symbology-style.db on the profile folder.

Same issue occurs in Pop! OS 21.04 after updating to QGIS 3.20.1. Resolved by deleting symbology-style.db in the profile folder of the current default profile (thank you @ondrolexa). Might be an issue with custom symbologies as @ulmi mentioned.

You don't need to remove the old symbology-styel.db file...

The only structural difference between the old and "new" symbology-style.db is the lack of a "stylemetadata" table in the "new" database.

If you delete this table from the old database, QGIS 3.20.1 will start and use all the current styles in the database.

I did this using DBeaver (you need to kill any running QGIS processes to unlock the database first).

@klakar

Thanks this resolved the issue for me on Windows 10
:)

Windows 10 and having the same no start issue. I also can no longer install an older version to run either.

completely shutting down seems to have fixed my issue.

This issue should now be resolved by #44343 and included in the next 3.20.2 release.
If you want to workaround the issue without using a database manager you can use the terminal/command line:

1. Open terminal in the default profile folder
2. Open the database:
   sqlite3 symbology-style.db
3. Drop the metadata table:
   drop TABLE stylemetadata;
4. Exit the database:
   .exit

Note! This is not a FIX, it's a workaround. The bug in the code is still there and you probably should be careful with what you do here. At least take a backup of the symbology-style.db file.

I think it is a big mistake not to immediately release the fix to this as 3.20.1-2. Don't wait for 3.20.2.

It is surprising to me that a mere one row database can stop the whole application! I find it even more surprising that customized styles causes this issue. While I do not remember when I created custom styles, it is quite baffling that this issue will affect users who are knowledgeable enough to create customized styles! Do not regular users of QGIS create customized styles for their own uses?

1. Open terminal in the default profile folder

Just a short note: you actually have a symbology-style.db in every profile folder, so I think you have to 'fix' the db in the profile folder(s) you actually use(d) last time you shutdown QGIS.
Also, if you can start QGIS from the command line, you can use qgis --profile foo to let QGIS create a fresh profile with a pristine symbology-style.db and start QGIS in foo profile.

3.20.1-2 is available on osgeo4w and msi.
Jef was building for Ubuntu and debian few hours ago so it should be available now (or very soon)

Just upgraded from 3.20 → 3.20.1-2 and everything worked as expected, however i haven't tested from 3.20.1 → 3.20.1-2 with the workaround @klakar mention!

Should the issue be closed since it has been fixed #44343 and the new packages addressing it have been release?

+1 for closing this issue.
I did upgrade from 3.20.1-1 and the original issue is resolve.
Any remaining issues should probably be treated as a new issue.