I get lots of errors like these:

A critical error has occurred.
Object reference not set to an instance of an object.

and the docs are not helpful in solving these. On one of my test portals, the problem started after I uploaded a new skin, which actually was a patched version of an existing skin. So I knew that "something" has to be wrong with my skin, but what? There is no help whatsoever in the documentation about how to search for the solution to problems like these. By the way, the problem persists when returning to the default DNN-blue. I can now no longer see or edit the contents of that portal.
In another thread, I read about different language packs being the cause of this error. However, I get the error when using US English.

On another portal (under the same installation of DNN) I get the above error when trying to log in as the administrator account I created for it. Logging in as Host works, but not with any administrators. Why? Again, there are no hints in the documentation on where to look, what to try, what to do when this error occurs. Does nobody else experience this?

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Groetjes, Pim