In rare cases, Kartris will seem to run correctly except for the auto-complete box for adding new category parents to a product, or the similar system for adding related products. Instead of returning suitable results, these instead appear to display the source code of an HTML page, one character per line.
In fact, what is happening is that the web service call used to create XML of results to pull into these boxes instead returns an error message. The AJAX code therefore sees this error page as the results set, and tries to format it as results.
Further investigation may reveal this or a similar error in the logs:
Exception message: Request format is unrecognized for URL unexpectedly ending in '/GetCategories'.
The cause appears to be a glitch on the server. The solution, in the case we experienced, was for the host to reset/revert the IIS handler mappings on the affected server.