Checkout button throws Internal Server Error

[aapw]

I am using zen-cart 1.5.5a

When clicking the checkout button I get this error:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator at [email protected] to inform them of the time this error occurred, and the actions you performed just before this error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

I have gone back and forth with my host provider and nothing they try works.
Does anyone have an idea how I can diagnose and fix this problem?

[lat9]

Check for myDEBUG*.log files in the site's /logs directory. That/those will identify the source of the Server 500 issue.

You might also want to sign into your admin and navigate to Tools :: Server/Version Info to see if your webhost has "helped you" by upgrading your site's PHP version. Zen Cart 1.5.5 should be able to run on PHP versions up to and including 7.1.

[aapw]

Thanks for your help!
I found a log file that gave me the answer.
For me it is the ups shipping module causing the problem.
Now I can get the most up to date version and update it.

Thanks again.

[mc12345678]

Thanks for your help!
I found a log file that gave me the answer.
For me it is the ups shipping module causing the problem.
Now I can get the most up to date version and update it.

Thanks again.

Should also be looking into upgrading the entire site and not just the one application.