So they updated from 5.7.x direcly to 5.8.x?

I tried updating a old test site. In "Application > Config" You will see that there is an file update.php if you remove that file it goes back to the original installation (rename that so you you put it back easily (BACKUP))

So in my case I updated from 8.1.x to 8.2.x. When deleting de "update.php" it went back to 8.1.x.

What info is in your "update.php"?
Maybe you can try that? I don't know any other way other then using backups.

Yes, they updated from 5.7 directly to 5.8.

Thanks for the suggestion. When I rename that file, I get an error on the site:

Class \Concrete\Core\Permission\Key\CategoryTreeNodeKey does not exist

Any ideas how to resolve that?

Searched a bit but could not find anything that could help you. I don't know how to resolve that error. (You can not go to dashboard to clear cache? sitename.xx/index.php/dashboard/system/optimization/clearcache)

Maybe someone else knows what the error means and how to fix it.

If I was you my last try would be to download the 5.7.x version en upload it then put back the maps: Application > Config, Application > Files, Application > Blocks, Packages.. and everything else that was changes by you. (remember to backup files etc. first)

Can't help you any further.

I'll try loading up the 5.7 files and see how that goes. Appreciate your suggestions.

Thanks for your suggestion. I was able to get this resolved by installing a fresh copy of 5.7.5.13
to another location and then copying and replacing some of those files to my corrupted site and using my database backup from a few days ago.

Found this:
https://www.concrete5.org/community/forums/installation/unexpected-e...

and can you tell me what is in your update.php?

I saw that too and just started copying the missing files over one by one when I encountered a new error and eventually landed on an error that was more than just a missing file. I have that update.php file renamed at the moment, but the contents of it are:

Seem nothing wrong with that file (just pointing to updated version 8.2.1) so if you delete/rename the file it just takes the original installed version... but then I don't know why you get that error and can't help you any further :(.