Multible Installations error after server update

we have 2 installations of PHPList 3.5.4 on our server in separate folders (lists1, lists2).
Now, after Upgrade from Apache 2.2 to 2.4 and PHP 5.3 to 5.6, strangely one of the installations always returns a server error 500 for all admin requests. It seems that after server restart, the first installation that is called works normal, while the one called second is somehow blocked. I could not to track this down, maybe it has something to do with the phpmailer class (which in this case is included from the distribution)?
Best regards

I can avoid the 500 error by inserting a die(“Test”); at certain points in the index file and several included files, however after the inclusion of the phpmailer class there was always a 500 error. Our server administration has no idea what could be the reason. Has anyone experienced similar problems or can give us a hint what to look at?

What do you mean by this?
You could try using PHPMailer 6 by setting a define for USE_PHPMAILER6 to be true.

I tried it, but still the second installation returns a 500 error. I asked about phpmailer, because before the inclusion of it, a die(‘Test’) is executed, which is no more the case afterwards.

@rkir Can you confirm exactly where the die() was added? The inclusion of PHPMailer is over several lines of code in file class.phplistmailerbase.php.

Also, is phplist returning any HTML? Even though an empty page is displayed you can view source to see any underlying HTML.

In class.phplistmailer.php is the last file where the die() output is displayed (no matter which line). In class.phplistmailerbase.php or later, the die() has no more effect. The result is a browser page with the message of http error 500.

@rkir In the config.php file do you have a custom location for phpmailer using PHPMAILER_PATH ?

No, like I said, it is included from the distribution.