This issue was fixed by updating VS2015 with Update 3. Thank you Hans Passant for pointing out this fix in the comments.
When editing C# comment blocks, I get the error listed above. This seems to happen only when the cursor is directly to the left of a comment block and pressing Enter to push the block down.
This GIF will probably be the best explanation. The error happens when pressing enter.
This does not happen if the cursor is moved even one character to the left or right of /* and only at the left margin. If there are no other characters within the comment block, just /**/, then the error does not occur. Regular comments are not affected. To be clear, the error only happens when pressing Enter at this location, with '|' designating my cursor:
|/* comment block here */
Similar error messages found online seem to have other causes, as their solutions do not seem to work for me.