QSM isn’t working at all anymore

I tried QSM and was happy with it, so I went ahead and purchased 3 addons to continue my development. Suddenly QSM doesn’t work anymore, trying any basic functions generates errors (‘pages’ is invalid index, Internal Server Error when it fails to create new questions). I am not using any other plugins, I even disabled the QSM addons. The only recent change I can think of is that my web host upgraded to WordPress 5.1.1 (it was already PHP 5.6). I have completely deleted the QSM plugin and reinstalled the latest version, and it does the same thing.

Examples:

Edit New quiz, add single page, click “Save Pages”:

[22-Mar-2019 13:49:44 UTC] PHP Notice: Undefined index: pages in D:\Hosting\2317187\html\wp\wp-content\plugins\quiz-master-next\php\admin\options-page-questions-tab.php on line 261

The admin panel msg does say “Questions and pages were saved!”, but I’m not sure what it actually does.
When I go back around and try to edit the new quizagain , it doesn’t seem to take any changes whether I’ve added or deleted pages, and shows up with random contents – sometimes no pages, sometimes 1 page, sometimes no questions, sometimes 9 questions (all “new questions”, if present).

Whenever I click “Create New Question” It just pops the error
“Internal Server Error! Please try again.” and no new questions show up. Until I go back around and edit the quiz again, as noted above, and then it randomly has 0, 1, 2, or 9 “new questions”. Sometimes I can delete the questions, sometimes not – whenever I re-edit the new quiz the contents seem random.

Another error that is showing up in the log:

[22-Mar-2019 15:03:48 UTC] PHP Notice: A feed could not be found at https://quizandsurveymaster.com/feed. A feed with an invalid mime type may fall victim to this error, or SimplePie was unable to auto-discover it.. Use force_feed() if you are certain this URL is a real feed. in D:\Hosting\2317187\html\wp\wp-includes\class-simplepie.php on line 1555

I am quite familiar with programming, and I’ve looked at the code some, but since there are multiple various errors I’m thinking there is some low level problem causing everything to fail. And QSM worked fine before.
The web host is GoDaddy, on IIS.

Can you help me?