Hi Kevin,
It is likely there is a special character somewhere in your file. If you are able to run the file without any issues, it is likely somewhere in the comments.
Attached is an example of an M file with a special character in its comments.
If you cannot find a special character in your file, another thing to check would be any places where "char" is called on a number. The way "char" function works is to create unicode commands of the numbers, so this is an easy way to generate invalid XML characters. For example:
char(20)
For converting numbers to characters it is recommended to use "num2str" instead of "char":
num2str(20)
