Migx configulator and connector.php 404 error

I am encountering the strange error in migx configurator.

  1. This only happens in my production site. Same configuration works fine on my test site. Versions of PHP (8.2), modx (3.04) migx (3.02 beta 1) are all the same, at least on the surface. But the production site is 10 years old. Many legacy elements and files no longer used are there.

  2. Problem in the form tab editor. In one of related database lookup, I have the following output option for a field author_id, input tv type set as listbox

@SELECT name , id from modx__db_author

This works fine. But as soon as I change it to

@SELECT name as ‘-Chose one-’ , id as 0 UNION SELECT name, id from modx__db_author

and save, I can not go back to form tab any more. It shows “loading” spinner forever. Looking at the Network tab of developer console of my browser,
I see connector.php post returned 404. First 200 response if from opening Migx configulator. Next 404 is from trying to open form editor from form tab
Screenshot 2024-02-13 133150

So the temporary solution is to revert back to original @SELECT binding (by direclty editing mysql table row’s formtabs field) and do away with ‘Chose One’ option.

I dont know what is a problem. Further more, I dont know why this is happening in one installation and not in the other. Could it be something more serious lurking in my production site? So any suggestion for what I should be looking in debugging this anomaly is appreciated. Thank you!