XF 2.2 Error: Invalid User ID after 1.5 -> 2.2 upgrade

[kontrabass]

Hi!

After performing the upgrade from 1.5 to 2.2, whenever I try to rebuild the thread cache etc I get a error:

Oops! We ran into some problems.​

Invalid user ID specified.

I'm wondering how I can get more specific information on this error (which user ID?). There is nothing in the error log... Any ideas?