Commerce form refresh: 500 error on /en/system/ajax Where can I find more information about this error? Nothing is appearing in logs, the error message itself has no useful data.

An AJAX HTTP error occurred. 
HTTP Result Code: 500 Debugging information follows. 
Path: /en/system/ajax 
StatusText: Service unavailable (with message)


Some background info:

Just grasping at straws this point, and may actually completely rework how this feature will work, but just in case anyone has any bright ideas:

Product variations exist in Commerce, certain options refresh the form via Ajax (this is a built-in commerce function), and on refresh any attribute/field that is no longer available is not included with the refreshed form. Again, total standard Commerce function.

I have used a few modules to help me create variations, such as Bulk Product Creation, and as recently has 1 month ago used the exact same process to do this successfully. That product is still working.

When I inspect the ajax call itself there's nothing really unique to this vs. the other working product(s).

I have tried eliminating variations, increasing PHP memory/POST stuff, etc., and have also turned on full error reporting.
Answering my own question so my stupidity is saved for all to learn from.

More details can be found in the "Reports" section of the site.

I have now resolved my issue, and it was a line of code in a custom module we made.