The problem is that you're putting something in session that isn't
serializable (as you suspect).
Since you've marked your own classes as
are a couple of likely possibilities:
You've inherited from something that isn't
(so marking your inherited class
You're attempting to store something in session that isn't one of
your classes, and isn't serializable, most likely a framework class. Common
examples might be
Also the exact class that is causing the problem should be clear from
the full exception info.
For example the following is in the error message shown in the question
[SerializationException: Type 'Gebruiker' in Assembly
'App_Code.qzuhycmn, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'
is not marked as serializable.]