Do you use the same internet connection on Linux desktop and on the phone or is the phone connected to a separate mobile network with a SIM card? If you can collect the logs from Android and send it to me or delta@merlinux.eu, that would be helpful.
Most interesting is a log of trying to create a new profile with nine.testrun.org. It could take several minutes until it fails, once it fails you can save the log in Settings → Advanced → View log.
To get longer logs on Android, here is the instruction:
There are two almost identical APKs at https://download.delta.chat/android/beta/deltachat-gplay-release-2.49.0.apk and https://download.delta.chat/android/beta/deltachat-foss-debug-2.49.0.apk, so you can setup a second one for testing and collecting the logs if you want to make sure the logs have no info about your existing profiles.