RSS:error

Hello

My RSS-feed has work without problems, but for two weeks I got this error message:

RSS-error: This XML document is invalid, likely due to invalid characters. XML error: Invalid character at line 199, column 31

I have two bloggs with the same RSS-feed and it works with the “The Blaskan Theme” but not with “The Twenty Twelve Theme”.

I use the WordPress Online (no local server) with following widgets:
Search, Categories, Follow this blog (entering your e-post etc.), RSS-feed, Latest posts and Archive.

What can I do to solve this problem?

Regards from PO