Interesting enough I am not able to replicate anymore the issue of not being able to see what I enter in answer field when MathQuill is enabled. I updated my Google Chrome this morning and the issue is gone. And that is currently not happening with my Windows laptop.
But the error message
"DBD::mysql::st execute failed: Incorrect string value: '\\xCF\\x80 \\x0965...' for column 'answer_string' at row 1 at /opt/webwork/webwork2/lib/WeBWorK/DB/Schema/NewSQL/Std.pm line 837"
still appears with Google Chrome for some problems but not all problems when copying and pasting the unexpected symbol '-'.
Thanks for the suggestion of trying to fix the issue by changing records in the database. I will try to do what you suggested with the MySQL commands you provided when I have a chance and will report here.