I finally managed some importing of certificates, using kleopatra. Don't know
why gpa rejected the import.

First I found an actual valid certificate in .cer format on
http://www.bundestag.de/adressbuch/search.do
(Please choose your favorite MdB :-) These days I'm not sure if she's too cool
or with 'them' in bed )
.CER binary files - I didn't get it how to convert it using GnuPG to .PEM,
kleopatra and gpa rejected it (0 keys found) "Der Zertifikats-Typ von
C:/Users/USER/Documents/MdB.cer lässt sich nicht bestimmen".

• however Windows does the job when opening the file using the builtin system

panel to export it to base64 encoded format and renaming it to .pem (gpa rejects
that too - kleopatra gets it).

Then there were three higher level certificates in DER format, found on
http://www.bundestag.de/bundestag/abgeordnete17/emailzertifikat/index.html
I could also not import them using gpa, however kleopatra did it.

Importing .crl files from the source linked on bundestag.de, doesn't work:
(1)
gpgsm: enabled debug flags: x509 mpi crypto memory cache memstat hashing assuan
gpgsm: DBG: connection to dirmngr established
gpgsm: response of dirmngr: BER Fehler
random usage: poolsize=600 mixed=0 polls=0/0 added=0/0

outmix=0 getlvl1=0/0 getlvl2=0/0

secmem usage: 0/16384 bytes in 0 blocks

(2)
gpgsm: enabled debug flags: x509 mpi crypto memory cache memstat hashing assuan
gpgsm: DBG: connection to dirmngr established
gpgsm: response of dirmngr: Unbekannter Systemfehler
random usage: poolsize=600 mixed=0 polls=0/0 added=0/0

outmix=0 getlvl1=0/0 getlvl2=0/0

secmem usage: 0/16384 bytes in 0 blocks

A minor issue was an error when updating X509 certificates, there showed up the
error that "C:\Users\USER\AppData\Roaming\GnuPG\policies.txt" could not be found

• creating an empty file made kleopatra to update "successfully".

Gruß