Because of the dmesg-Message 
[    9.557455] cfg80211: loaded regulatory.db is malformed or signature is missing/invalid

I rechecked the regulatory.db and did found 2 more files to install 
1.) new pem-certificate ( <-- for signature )
2.) new .bin

In the archive 
http://ftp.be.debian.org/pub/software/network/wireless-regdb/wireless-regdb-2019.03.01.tar.gz
you could find the files
regulatory.db
regulatory.db.p7s
regulatory.bin
sforshee.key.pub.pem

copy 
from the archive directory wireless-regdb-2019.03.01 to /lib/firmware 
regulatory.db        to /lib/firmware 
regulatory.db.p7s    to /lib/firmware 
regulatory.bin       to /lib/crda
sforshee.key.pub.pem to /lib/crda/pubkeys/

(reboot)
[    9.519254] cfg80211: Loading compiled-in X.509 certificates for regulatory database
