INVALID_CHARACTER_ERR: An invalid or illegal XML character is specified.

I have a stored procedure in my Azure SQL database that returns a set of data. This has worked flawlessly for a couple of years and I've made no changes to the code in at least a year.

I am now getting this error in the database-connector error log:

       INVALID_CHARACTER_ERR: An invalid or illegal XML character is specified.

It is only returning this error for a couple of locations (table holds data for specific work locations). All other locations work perfectly.

I've checked the stored procedure in my SQL tool for the offending locations and it works fine. I've checked the freeform text areas for text that could mess up XML formatting and don't see any. I've also run the SQL from the stored procedure that returns results to have it returned as XML against the specific schema -- it returns the data in the formatted XML just fine.

I'm at a loss to explain or fix this one.

Any thoughts? Any other data from the error log needed?