MTurk HIT page not found when submitting

Whenever Turkers submit their assignment, they receive the error message:

"Sorry, we couldn't find that page"

When opening the web browser console, one Turker said they received this error associated with the POST from pressing submit:

"Uncaught SyntaxError: Unexpected token '&' (at submit:26:52)"

The HIT generally seems to work properly, even flashing "Loading next HIT" very briefly after submitting, but then posting the above error.

Any ideas how to fix this issue? Thanks!