It is likely an ''illegal'' character in one of the MEMO fields (Comments or Description are usually offenders).
Sometimes people copy paste bad characters as part of something from Outlook or foreign languages. Sometimes we see where in the defect the tester is actually discussing some bad characters they are having issues with in the AUT (application under testing).
Anyway, if you can isolate the Defect ID, you can either delete it from teh back-end BUG table for the project, or have the DBA help you REPLACE the Notes or Description field contents wtih something else.
I have used Notepad++ to examine the field contents -- copy contents to clipboard/paste into notepad++, then select option to show all non-printing characters and they show up as black squares. Some illegal characters we often see are EOT, EOM (CR and CR-LF are ok.)
It may be possible to delete the ''offending'' characters and replace the text, then close any user connections to the project and try again (cycle service if needed).
Another thing to try would be to replace the description text, for instance (in the Database) with VERY simple text like a short phrase to see if it clears up the issue, then you know ti was the original field contents.