If you are getting 500 Internal Server Error in Mautic, then don't consider this to be a bug with Mautic. This error is mostly because of some misconfiguration on your server or Mautic instance.
In this tutorial, you will learn a few of the most common reasons for getting, 500 Internal Server Error in Mautic.
It may be a temporary issue because of the memory consumption during the upgrade or optimizations of segment queries. If you check the logs, then you might find an error something like this;
mautic.ERROR: Segment QB: Query Exception: An exception occurred while executing 'SELECT count(leadIdPrimary) count, max(leadIdPrimary) maxId, min(leadIdPrimary).
Refer this Stackoverflow post for more details: https://stackoverflow.com/questions/4380813/how-to-get-rid-of-mysql-error-prepared-statement-needs-to-be-re-prepared/4381155#4381155
In case, you are getting some error like the one below, then most probably this is because CURL module is not installed or not enabled in your PHP.
[2016-08-15 19:11:52] mautic.CRITICAL: Uncaught PHP Exception Symfony\Component\Debug\Exception\UndefinedFunctionException: "Attempted to call function "curl_init" from namespace
In case you are getting some error like the one below:
[ssl:warn] [pid 3609:tid 139967997105984] AH01909: localhost:443:0 server certificate does NOT include an ID which matches the server name
This means there is some misconfiguration in your Mautic setup.
Hope the above solutions helped you debugged the issue. In case you have discovered something, which is not covered as a part of this tutorial, then please feel free to contribute those below in comments.