Internal Server Error

I was running PL 3.3.1 and after creating a new message and testsending it for the first time an ‘Internal Servcer Error’ occured ( Internal Server Error / The server encountered an internal error or misconfiguration and was unable to complete your request. / Please contact the server administrator at to inform them of the time this error occurred, and the actions you performed just before this error. / More information about this error may be available in the server error log.)

Unfortunately this happens again and again, even after Updating to 3.4.1. Usually the server responds again after 10–15 minutes just to fall into that error after 5–10 klicks.

I tried PHP 7.1, 7.2 and 7.3 without experiencing any difference. If it helps I could provide with all available informations form phpinfo.php. The error log states: 29.04.2019 09:30:54 [client] End of script output before headers: index.php

The provider is ‘Strato’ in Germany–regularyly not regarded as the best, I know.

Is there any idea out there? Thank you.