Installation behind nginx rev. proxy fails

I installed passbolt on a debian 9 running with apache 2.4 and php 7.3.
Connection from the public is realized through a reverse proxy with nginx.
The healthcheck fails with the following result:

[PASS] Full base url is set to https://passbolt.freifunk-stuttgart.de
[PASS] App.fullBaseUrl validation OK.
[FAIL] Could not reach the /healthcheck/status with the url specified in App.fullBaseUrl
[HELP] Check that the domain name is correct in config/passbolt.php
[HELP] Check the network settings

SSL Certificate

[FAIL] SSL peer certificate does not validate
[FAIL] Hostname does not match when validating certificates.
[WARN] Using a self-signed certificate
[HELP] cURL Error (7) Failed to connect to passbolt.freifunk-stuttgart.de port 443: Connection refused

Some of the checks are misleading: The LE cert for https has the correct hostname and P. 443 is available. Although I am shure I have a correct apache *.conf , the main url answers with “the requested URL was not found on this server.” Hmmmm …

The only similar topic I can find is at Install passbolt behind an existing Nginx Reverse Proxy but its not really helpful.

I would appreciate if someone can give me an idea where to search for the error. I believe it has to do with the reverse proxy or the cert which is provided there…

Thanks, Thommie