error http 500 when clicking Contribute button
I don't believe that the error shown in the question is likely to be the one that triggers the error 500 for your registrants. Note that the backtrace shown starts with a CLI entry point civicrm_cli-&gt;callApi(), suggesting this has been triggered from system cron on the commandline, while your issue is reported as occurring via the web.

First, find the relevant error log.

If you are seeing error 500s, you need to understand what the fatal error was. This probably involves inspecting your webserver error logfiles. You can find these via the webserver (eg Apache / Nginx / ...) configuration, and you may also find there are separate logfiles for a separate PHP process.

The PHP error logs may be identified via phpinfo() (look for "error_log" setting). Webserver logs you'll need to ask your sysadmin or hosting provider. Bear in mind that a shared hosting provider may be reluctant to give access to logfiles which contain information from other sites, but you will need that information to resolve this issue without guesswork.

There should be a log entry made at the specific time you see the error in the browser. Don't accept any red herrings until you find out what this is, because it's essential to you finding the source of the problem.

If your hosting provider cannot help you track down CiviCRM errors, consider finding one who does: https://civicrm.org/partners