why does postgres complain about "ca md too weak" when connecting with ssl

I'm trying to setup a postgres server (version 10.4) with openssl access only. I created a self signed ca and the necessary private and public keys for the user and the server with easyrsa version 3 as described here and put it in the directories as stated here. Afterwards I added this line

hostssl  all  myname  192.168.0.0/16  cert

to the pg_hba.conf as described here and here But when I try to connect to the database with

psql -h 192.168.0.222 "sslmode=verify-ca sslcert=myname.crt sslkey=myname.key"

this error message pops up

couldn't read certificate "myname.crt": ca md too weak

and in the server log

couldn't accept SSL-connection: success

(maybe the messages are not literally correct, I translated them from German)

That is funny, because when I look at my certificates with

easyrsa show-cert myname

I see the following lines (and more):

Certificate: 
   Data: 
   Version: 3 (0x2) 
   Serial Number: 
       don't show it here
   Signature Algorithm: sha256WithRSAEncryption

and the same for my root certificate ca.crt and the certificate for the postgres server. So obviously the certificates are signed with sha256 method and not with md
What is even more funny, is that it worked in the beginning, and some days later, when I tried it again the above mentioned error message appeared and I'm quite sure that I didn't change any configuration meanwhile. Maybe some update of openssl or postgresql was in between.