Ignore the line 720 error. You should care about "code on line 23". That means there is a plugin layer that has an error in line 23. Look for enabled plugins only, then open qa-plugin.php for each and check the layers being loaded and one by one look for line 23.
Alternatively, if you'are using a good IDE, just look for the red spot in the file tree :)
In this case, you seem to have the word "question" written in that line, but it is not a string. That's generating the error.