I figured out what the issue is with your connection. By default the OpenVPN for Android app expects to see the ciphers
AES-256-GCM
AES-128-GCM
CHACHA20-POLY1305
This list is the default set of ciphers for OpenVPN, which the app uses.
So when I tried to make a connection using the same settings as you had on your main screen, ie with the cipher AES-256-CBC selected then the OpenVPN for Android refused to make a connection because it didn’t get offered a cipher in its default list.
You must have then edited the profile in the app by going to the tab labelled AUTHENTICATION/ENCRYPTION and in the encryption section there is an option labelled Encryption ciphers which would have been marked as Not set. This means that the OpenVPN for Android app will only accept one of its default ciphers.
I suspect you then added into that option AES-256-CBC but without also specifying the above default ciphers so that AES-256-CBC will then be the only cipher that OpenVPN for Android will accept.
You should enter into that option the following
AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305:AES-256-CBC
This will then keep the openvpn default ciphers and add your AES-256-CBC cipher into the client allowed list.
Then when you connect with the CU197 it will then do negotiation and find that it has three ciphers that overlap and will choose the strongest of those three.
If you need to go back to accessing your CU196 system then the entry for AES-256-CBC will allow that old version to work.
EDIT:
I just took the IPFire CU196 using the AES-256-CBC cipher with the OpenVPN for Android app with the 4 ciphers shown above entered into it. This allowed the AES-256-CBC cipher to be accepted and I got a working connection.
I then updated to CU17 and cipher negotiation occurred between the android app and the CU197 IPFire system and AES-256-GCM was selected and gave me a working connection. So I have been able to verify that my suggestion of adding the 4 ciphers into your android app worked.
I would suggest that once you are fully running with released CU197 that you should clear all the ciphers from the Encryption section on the android app. That way the app will always use the latest default ciphers from OpenVPN, which will also be the case with IPFire. That way in the future negotiation will always successfully find a common cipher that is the strongest available.