Network Error: Chain validation failed

My Setup: NextcloudPi Installation on Debian - reachable via freedns-domain, Portforwarding on 443 and 80

My Problem: Passman Android App wouldn't connect to my Nextcloud instance due to "Network Error: Chain validation failed"
Everything else ist working fine, Chrome Extension or normal Access via Browser.

Lets Encrypt Certificates got automaically installed via NextcloudPi's Adminpanel Letsencrypt-App (Certbot?)

I Solved the problem following this instructions, but it might be only temporarily and will be overwritten when the new Certificates are generated.

nano /etc/apache2/sites-enabled/nextcloud.conf

then replaced
SSLEngine on
SSLProxyEngine on
SSLCertificateFile /etc/letsencrypt/live/mynextcloud.domain/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/mynextcloud.domain/privkey.pem

with

SSLEngine on
SSLProxyEngine on
SSLCertificateFile /etc/letsencrypt/live/mynextcloud.domain/cert.pem
SSLCertificateChainFile /etc/letsencrypt/live/mynextcloud.domain/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/mynextcloud.domain/privkey.pem

Maybe Passman is missing support for the Combined Certificates? Could this be added in a future Release, since this is the standard letsencrypt way of installation? This would be great!

I hope my fix helps other

Start with the reported TLS configuration in /etc/apache2/sites-enabled/nextcloud.conf and compare the fullchain.pem setup with the cert.pem plus SSLCertificateChainFile setup. Reproduce the connection from the Passman Android app against a NextcloudPi instance using the standard Let's Encrypt configuration, then verify that the certificate chain is accepted without manual Apache changes.