Hi Bogey71,
I encountered this problem too today, and I found out the way to solve it.
Execute ckan.exe from command window, with option "update" (like this: ckan.exe update).
You will get a message that says some modules are updated, and then you can use ckan normally (from GUI too, of course).
See this issue: https://github.com/KSP-CKAN/CKAN/issues/1982
Best wishes,
N. Kaitoh