You do not say if the error is on serialization or deserialization or in the designer.
So if it is
deserialization.
This sort of error did occur if the version number of your assembly changed between serialization and deserialization. i.e. you serialize, then do a build, then deserialize.
Although I thought that the problem had been resolved.
The way to test this is to run your app, close it, then run it again. If you still get the error then this is not the problem.
If you don't get the error then run your app, close it, do a build, run it again. If you get the error then this is the most likely cause.
The solution is to have a fixed version number. (In AssemblyInfo.cs ensure that the version number looks like 1.0.1.0 rather than 1.0.1.*.
Obviously your numbers might be different from above example, the important thing is that there are no asterisks.
If it is in the
designer,
this[
^] might help.