Here is the answer:
https://github.com/mattermost/focalboard/issues/2119#issuecomment-1013751739

@robi It's not an issue with cloudron. So I guess as soon as there is a fix by Mattermost it will also land here.
The only action step from @nebulon and cloudron would be to add some tests for boards in future releases.

focalboard_schema_migrations

Was having the same issue and this seemed to work. TLDR; Seems like I only had to update dirty.

Before Mattermost update:

select * from focalboard_schema_migrations;

+---------+-------+
| version | dirty |
+---------+-------+
|      14 |     0 |
+---------+-------+

After Mattermost update:

select * from focalboard_schema_migrations;

+---------+-------+
| version | dirty |
+---------+-------+
|      15 |     1 |
+---------+-------+

Then I ran the following:

update focal_schema_migrations set dirty=1 where dirty=0;
commit; 

Then I rebooted the Mattermost instance and checked the table again:

select * from focalboard_schema_migrations;

+---------+-------+
| version | dirty |
+---------+-------+
|      16 |     0 |
+---------+-------+

Finally, I restarted Mattermost Desktop and checked that I could load Boards again, and it all works great.

• version 15 -> 11 (I figured that's the current version)
• dirty 1 -> 0

Then I found this in my logs:

Jan 18 11:25:23 {"timestamp":"2022-01-18 10:25:23.926 Z","level":"error","msg":"Not activating plugin because diagnostics are disabled","caller":"app/plugin_api.go:928","plugin_id":"com.mattermost.nps"}

So I enabled diagnostics. However this was the result (not sure if its related to this update):

Jan 18 11:28:35 {"timestamp":"2022-01-18 10:28:35.229 Z","level":"error","msg":"Unable to activate plugin","caller":"app/plugin.go:142","plugin_id":"focalboard","error":"error initializing the DB: Dirty database version 15. Fix and force version."}

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