is this a limitation of the server or of nextcloud version?
Is it only a missing cipherspec or another tls-version?
do I have a chance to fix this with my (very) old android version, which I can not leave at the moment
Yes, you have to set up TLS ciphers on the Web server (e.g. nginx, not Nextcloud) which are available on your Android device. See https://developer.android.com/reference/javax/net/ssl/SSLEngine for a list.