I upgraded my forum from version 4.1 to a higher one and now I'm unable to log into my Admin CP because it says the following error:

I cannot even see what version number my forum is currently on. How do gain access to the Admin CP now? I tried in multiple browsers and it gives the same error, so it's not a browser issue.

It looks like you're still using 4.1, but that's a front-end error. You may be able to access the AdminCP by going to <you-community-url>/admin

You will still need to upgrade to the latest version however.

 

Exact same error happens in the /admin

I'm able to access your AdminCP login page without issue (We can't log in because we don't have any credentials on file).

The best solution however is to get upgraded to the latest version, the document I linked to has the instructions to manually upgrade.

Additionally, you can use this file to confirm your server is ready to upgrade to the newest release.

Upload it to your community folder and visit it in your web browser.