XML comments can't contain --

Cause:

Comment data may not contain two hyphens next to each other. A sequence of two hyphens must only be used to open or close a comment.

Example:

Good<!-- comment -- other comment data -->
Good<!-- comment --- other comment data -->
Good<!-- comment - other comment data -->
Good<!-- comment - - other comment data -->

Solution:

Remove or replace all occurrences of two sequential hyphens within comment data.

References: