Q2A works perfectly fine with PHP 7.4. In fact, that's probably the best PHP version to use 1.8.6.
If you're facing issues it is most likely related to a plugin. Try removing all plugins (except from the ones that come with Q2A) and retest.
For the first error, I'm under the impression the test is related to closing/opening questions.
The second error, seems to be related to session management. It seems to be trying to get a session from a deleted user. Most likely you have deleted a user that was navigating the site. You can disregard that one.