Recursive fallback not allowed for character \u003F.

Hi, 
My asp.net website suddenly showed the error message listed below. I Googled but didn't find any effective information.  The hosting company claimed that they didn't change any set up on the server. Now I'm runnning out of ideas :(.
Could anybody kindly provide some suggestions on how to deal with it? I would like to spend some time on it however right now I don't know where to focus.
Thanks in advance! Any thoughts/ideas/examples etc would be greatly appreciated. 
Gabriel
 
 
Recursive fallback not allowed for character \u003F.
Parameter name: chars Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in ...