Did I break my Ubuntu GPG installation?

[Stefan Boehringer]

Hello there.

I'm quite new to GnuPG. In November I played around with it and
generated my first key.

In the meantime I started to read more about it and decided to start
anew, generating a masterkey under more secure conditions (I used
Tails), keeping it offline afterwards and generated signing and
encryption subkeys for daily use.

The problem now is as follows: on my Ubuntu machine I deleted my old key
and imported the new subkeys. But GPG wouldn't let me en- or decrypt
anything. So I thought, maybe it would be a good idea to uninstall GPG,
delete the .gnupg-folder and install it again. I did that and imported
the subkeys, but it still doesn't work. The error is as follows:

>> gpg: Auf geht's - Botschaft eintippen ...
>> test
>> gpg: Keine gültigen OpenPGP-Daten gefunden.
>> gpg: processing message failed: Unbekannter Systemfehler

Could translate to: "no valid OpenPGP-Data" and "unknown system error"?

But gpg --edit-key stefan.boehringer at posteo.de shows me:

>> gpg (GnuPG) 2.1.15; Copyright (C) 2016 Free Software Foundation, Inc.
>> This is free software: you are free to change and redistribute it.
>> There is NO WARRANTY, to the extent permitted by law.
>> 
>> Geheimer Schlüssel ist vorhanden.
>> 
>> pub  rsa4096/98723XXXXXXXXX
>>      erzeugt: 2017-01-11  verfällt: niemals       Aufruf: SC  
>>      Vertrauen: unbekannt     Gültigkeit: unbekannt
>> ssb  rsa4096/42B4XXXXXXXXXXX
>>      erzeugt: 2017-01-11  verfällt: niemals       Aufruf: E   
>> ssb  rsa2048/6E101XXXXXXXXXX
>>      erzeugt: 2017-01-11  verfällt: 2019-01-11  Aufruf: S   
>> ssb  rsa2048/209F5XXXXXXXXX
>>      erzeugt: 2017-01-11  verfällt: 2019-01-11  Aufruf: E   
>> [ unbekannt ] (1). Stefan Böhringer (born Oct. 29. 1980, Regensburg, Germany) <stefan.boehringer at posteo.de>

I don't know why so much is stated as "unbekannt = unknown"...

On another Arch-Installation and on my Android-Phone using OpenKeyChain
it works just fine. What could I do?

Best regards
Stefan