symfony services – How to find the cause of “Uncaught PHP Exception”

When I go to any page, the following error appears in the log:

[proxy_fcgi:error] [pid 20333] [client] AH01071: Got error 'PHP message:
 Uncaught PHP Exception Symfony\Component\Routing\Exception\MethodNotAllowedException:
 "" at /var/www/html/web/core/lib/Drupal/Core/Routing/MethodFilter.php line 46', referer:

At first I thought it had something to do with my new theme that I am about to make. But if I change theme, the error is still there. I am using Drupal v9.2.5.

How do I track down the cause of the error?