I followed your steps and was able to reproduce the error for just the ApplyClass dropdown by clicking on it using the Fiddler tool.
Could you send us a screenshot of your errors from Fiddler as well as some more details on the errors you get with custom skins?
If you could send us a sample running
app reproducing this issue in a support ticket, it would really be most helpful.
Thanks for your understanding and looking forward to your reply.
the Telerik team