We've hit a pretty major issue with the combination of the WebDataTree and WebDataMenu.
Please see the attached solution; to reproduce the error, all you need to do is the following:
The error in the subject of this post will then occur. This can also be reproduced by clicking the standard asp.net button at stag 4 instead of repeating the right click procedure.
This only happens when NodeExpanded is set to Async. If you set NodeExpanded to On then this issue will not happen!
N.B. I've had to remove the ig_res folder to get the file small enough to attach.
The button that was clicked is there to cause a postback. It has no other intended purpose. It's there for the demonstration of the bug at step 4 of my instructions as per my description.
I tried out your instructions and was able to reproduce the error. Was that button being clicked supposed to do nothing? I have logged an internal bug for this issue and it will be investigated. If you would like to contact developer support, the bug number to reference is 20502.
regards,David Young