Please check if your characters are supported:
http://helpx.adobe.com/acrobat-com/formscentral/help/formula-syntax-built-in-functions.htm l "Unicode Character Support" section.
If they are, please provide more details of the type of form fields that you are using and the characters that you are inputting.
Are you inputting characters by doing a copy/paste from other source or typing?
I'm having the same problem with my Adobe FormsCentral Web Form. I have the form embedded on my website, and when someone tries to copy and paste text into, they get this error message: The field contains unsupported characters. Please modify your entry. Please help!
I've seen similar errors in other web-based applications.
If respondents are copying and pasting from MS Word into your form, that might explain part of it. When copying from a Word document, a lot of hidden formatting is carried along with the copy and paste operation. If they are using Word, try having the person save their Word document as a plain text file and then doing the copy and paste from that. That might at least point to what is going on.