Hello Marco,
Thank you for providing me a clear test case of the issue.
I successfully reproduced it and it looks like a bug - so we will research it and log it for fixing.
Let me suggest you overcome the issue by disable the RadComboBox item at client-side in this manner:
You will not need this workaround once we fix the issue.
Thank you for taking your time to report this - please find your Telerik points updated.
Kind regards,
Kalina
the Telerik team