There are some edge cases where something is broken and Whoops kicks in but then all the debugging information it gives is about itself breaking and not the root cause. If you turn it off then you get some more meaningful stacktraces.

There are multiple failures, this issue fixes one of them but not the ones below which I'll turn into another tracker once I can reproduce it properly.

eg I have gotten this error:

Fatal error: Uncaught Whoops\Exception\ErrorException: Undefined array key 1 in /var/www/master/vendor/filp/whoops/src/Whoops/Exception/FrameCollection.php:113 Stack trace: #0 /var/www/master/lib/setuplib.php(504): Whoops\Run->handleError(2, 'Undefined array...', '/var/www/master...', 113) #1 /var/www/master/vendor/filp/whoops/src/Whoops/Exception/FrameCollection.php(113): default_error_handler(2, 'Undefined array...', '/var/www/master...', 113) #2 /var/www/master/lib/setuplib.php(392): Whoops\Exception\FrameCollection->offsetGet(1) #3 /var/www/master/vendor/filp/whoops/src/Whoops/Handler/CallbackHandler.php(50): {closure}(Object(moodle_exception), Object(Whoops\Exception\Inspector), Object(Whoops\Run)) #4 /var/www/master/vendor/filp/whoops/src/Whoops/Run.php(398): Whoops\Handler\CallbackHandler->handle(Object(moodle_exception)) #5 /var/www/master/lib/setuplib.php(443): Whoops\Run->handleException(Object(moodle_exception)) #6 [internal function]: default_exception_handler(Object(moodle_exception)) #7 {main} thrown in /var/www/master/vendor/filp/whoops/src/Whoops/Exception/FrameCollection.php on line 113

After turning off whoops I get this much more useful error:

 
Exception calling 'tool_redirects_after_config' * line ? of unknownfile: call to default_error_handler()

• line 89 of /admin/tool/redirects/classes/regex_validator.php: call to preg_match()
• line 62 of /admin/tool/redirects/classes/regex_validator.php: call to tool_redirects\regex_validator->validate_syntax()
• line 82 of /admin/tool/redirects/classes/helper.php: call to tool_redirects\regex_validator->__construct()
• line 52 of /admin/tool/redirects/classes/helper.php: call to tool_redirects\helper::build_rules_from_config()
• line 102 of /admin/tool/redirects/classes/helper.php: call to tool_redirects\helper::get_all_rules()
• line 41 of /admin/tool/redirects/lib.php: call to tool_redirects\helper::redirect_from_rules()
• line 1152 of /lib/setup.php: call to tool_redirects_after_config()
• line 246 of /config.php: call to require_once()
• line 94 of /admin/index.php: call to require()
   

Fatal error: Uncaught coding_exception: Coding error detected, it must be fixed by a programmer: PHP catchable fatal error (Cannot modify header information - headers already sent by (output started at /var/www/master/lib/weblib.php:3340)) in /var/www/master/lib/setuplib.php:508 Stack trace: #0 [internal function]: default_error_handler(2, 'Cannot modify h...', '/var/www/master...', 439) #1 /var/www/master/lib/setuplib.php(439): header_remove('Content-Length') #2 [internal function]: default_exception_handler(Object(moodle_exception)) #3 {main} thrown in /var/www/master/lib/setuplib.php on line 508

The real issue here isn't important (was a php8 deprecation in a plugin). I also get a similar issue with whoops when you checkout a branch a slightly old branch which emits some debugging combined with an exception. Without whoops I get useful errors:

Hook callback definition contains invalid 'callback' class name in 'tool_mobile'. Callback class 'tool_mobile\hook_callbacks' not found.

and then later down the page is the exception:
ERROR!!! The code you are using is OLDER than the version that made these databases!

More information about this error
Debug info:
Error code: downgradedcore×Dismiss this notification
Stack trace: * line 536 of /admin/index.php: moodle_exception thrown
 

But if I turn whoops on then I get the meaningless error:

Fatal error: Uncaught Whoops\Exception\ErrorException: Undefined array key 1 in /var/www/master/vendor/filp/whoops/src/Whoops/Exception/FrameCollection.php:113 Stack trace: #0 /var/www/master/lib/setuplib.php(504): Whoops\Run->handleError(2, 'Undefined array...', '/var/www/master...', 113) #1 /var/www/master/vendor/filp/whoops/src/Whoops/Exception/FrameCollection.php(113): default_error_handler(2, 'Undefined array...', '/var/www/master...', 113) #2 /var/www/master/lib/setuplib.php(392): Whoops\Exception\FrameCollection->offsetGet(1) #3 /var/www/master/vendor/filp/whoops/src/Whoops/Handler/CallbackHandler.php(50): {closure}(Object(moodle_exception), Object(Whoops\Exception\Inspector), Object(Whoops\Run)) #4 /var/www/master/vendor/filp/whoops/src/Whoops/Run.php(398): Whoops\Handler\CallbackHandler->handle(Object(moodle_exception)) #5 /var/www/master/lib/setuplib.php(443): Whoops\Run->handleException(Object(moodle_exception)) #6 [internal function]: default_exception_handler(Object(moodle_exception)) #7 {main} thrown in /var/www/master/vendor/filp/whoops/src/Whoops/Exception/FrameCollection.php on line 113