Hi,
I had the same error. Is the associated entity generalized or specialised?
In our case, we had 3 columns over association. 2 of them retrieved the value over a specialised object, but the third over the generalised which caused the error.
Same issue, thought it was an access right issue as removing an association to a system entity resolved this issue at first but it occurred again after a custom search.
In my Case: In the retrieve of the main entity I GetCreate an Associated Entity in which I set several attributes with values I need to display.
I ended up using Jelle's approach to resolve the error.