I added a new check to the compatibility checker - it now knows how to look for illegal characters - such as those returned by calling the =CHAR() function with values lower than 32.
Because this test can be time consuming and is very rare, I also added a checkbox (called "Complete Analysis") to the ribbon that is turned off by default. Only when it's turned on will the new test become operational.
The download is available at the download-page.
Edit: Forgot to mention that this improvement was suggested by Andy Santos. Thanks Andy.