Upgrade 8.5.9 to 9.1.2 fails

I uninstalled all extra extensions to make sure the upgrade runs through. Unfortunately it does not.

I tried to replace the /concrete directory and after this failed (and I restored the original filesystem without extensions and DB) I tried the internal upgrading process:
Both ways show up kind of the same error message: Sooner or later some column is missing from an SQL statement.
Also: The internal process shows up a “bad gateway” error on the server. Refreshing that page does not change anything (bad gateway again). Afterwards the missing column error occurs.
Currently I restored the original fileset and DB (without extensions) so I do not have the exact message to post here.

All ConcreteCMS caching is currently off.
PHP 7.4.30 and MySQL version: 8.0.20 are installed.
PHP memory limit: 512 MB,
PHP max execution time: 30s


When I try to upgrade 8.4.3 to 9.1.2 I get:

An exception occurred while executing 'SELECT t0.siteID AS siteID_1, t0.pThemeID AS pThemeID_2, t0.pThemeSkinIdentifier AS pThemeSkinIdentifier_3, t0.siteIsDefault AS siteIsD  
  efault_4, t0.siteHandle AS siteHandle_5, t0.siteTypeID AS siteTypeID_6 FROM Sites t0':                                                                                          
  SQLSTATE[42S22]: Column not found: 1054 Unknown column 't0.pThemeSkinIdentifier' in 'field list'  

Adding that pThemeSkinIdentifier column to the database doesn’t seem to help, it still can’t find it.

Same problem here (but not the only one)
The problem is that the migration script skip the pThemeSkinIdentifier field before the " ```
An exception occurred while executing 'SELECT t0.siteID AS siteID_1…" part comes.

The recommended process is to update to latest 8.5.12 then update to 9.1.3. Have you tried with that intermediate update step?