We are making progress - as the error is now occurring somewhere else, and the message jogged my memory.
You can probably avoid the issue by not using "not equals" or any other UTF-8 as part of an answer... but PopUp will not allow you to use "math" inside the options.
I hope we can instead find the "fixes" you need to solve the problem - and that it will help make sure that the future WW 2.16 (if not 2.15) is really ready for UTF-8 without all these bugs. (It will still probably require the manual patch to Lite.pm as part of the installation procedure.)
UTF-8 support in WW was only added in WW 2.15, and there were issues none of us found during the testing before WW 2.15 was released. For a good while, I was aware that some problems could not be loaded in the library browser or via html2xml in general, but discovering why and finding a fix was quite challenging. We also told students not to use images mode due to one of the issues (see below). I hope that the patches discussed below also fixed the issue you are encountering now.
The "not well-formed (invalid token)" issue seems to have been related to problems where the correct/student answers could contain UTF-8 strings, as the "XMLRPC" system was not properly encoding strings deep inside the data it was passing, in particular for "answer" strings which contained UTF-8 characters. The "not equals" sign would almost certainly trigger that problem (in html2xml and apparently for the "randomization" button).
If my analysis is correct, then I hope that the changes made in the pull request https://github.com/openwebwork/webwork2/pull/1089 which had been merged into the develop branch will solve the problem you are having. I have been using that patched code for a good while, and it seems to solve the issues I had before with some problems in the library browser and with the same problems embedded via html2xml which I ran into before the changes. Also there was a second issue with UTF-8 answers discussed in https://github.com/openwebwork/pg/issues/443 and likely to effect your problems. The second issue was a failure to generating the answer table in "images" mode for cases like this. Students would only get an error page and not the feedback, etc. Davide Cervone proposed a fix, and Mike made the fix in https://github.com/openwebwork/pg/pull/466 which is also only in "develop" and not WW 2.15. Since that issue will effect such problems when used by students, if they change to using "images" mode for equation display - it is also something you probably want to consider patching on your system.
Sorry, I should have recalled both these issues when I posted my first response. Hopefully, we will not discover another problem later on.