Something extremely weird is going on with your site. I cannot think how normal execution of the code could produce the symptoms you are seeing, (even allowing for the type of errors that might be caused by typical bugs). The only way you can get to the "This choice was deleted after the attempt was started." message is if the question definition currently in the database is different from how the question was when the quiz attempt was started.
Have you looked through the server logs, and the Moodle logs, to find all accesses to this quiz, and the questions in it?
What caching have you got set up (a particular candidate would be the set-up for the 'Question definition' cache). If that cache is set up wrong, then I suppose that might possibly lead to these symptoms.