I've been able to reproduce WICKET-5898 with a test case and I've committed that to master to share the code. The failing test cases are @ignored, so to see the issue in action you have to remove that @ignore annotation.
It is a regression from Wicket 6 and was triggered (in our case) by an auto-TransparentWebMarkupContainer addition due to a src= attribute inside a TransparentWebMarkupContainer. Normal page renders work, AJAX requests fail in particular circumstances. I have tried to debug this part, but I'd like to continue testing our internal application with Wicket 7 so have to give up on fixing this one for now. Martijn
