Android App cannot connect, IOS App and Webclient working

Hi Community,
I installed a fresh server on Centos 8.1, everything is working so far.
When we tested the apps we recognized that the Android app cannot connect to server.
it looks like the app is using old ciphers which are no longer mentioned to be save.
We have a security device which holds the certificate and terminates the https session port 443 (Fortinet with a virtual server configuration) which forwards the traffic to the Server in the DMZ to port 3000 with http.

Message on the Android app is: (german: Hoppla! Die eingegebene URL ist ungültig. Überprüfen Sie es und versuchen Sie es bitte erneut!)
Translated: Ups. The URL is wrong chekc it and try it again.

I found a post where someone had the same problem with the windows app.
browser android and IOS was working but the windows app could not connect.
He wrote that the ciphers where too old. i assume the ciphers inside the Android App are to old.

Android App Version:4.6.4.2118 on a Samsung handy.
A solution would be to allow unsecure ciphers on the firewall which is a bad idea.
Any better ideas?

Greetings