OK, I checked one more time with:
Stretch (OpenSSL 1.1.0l)
Ubuntu 18 (OpenSSL 1.1.1)
Buster (OpenSSL 1.1.1d)
The letsencrypt certificates for the panel (Panel URL & imscp_services) are issued without any problems ...
But all LE certificates for the customers are shown as invalid (verification process fails)
For me the same.
From my all domain (wich had ssl conf in the apache directory), now there is no _ssl.conf file. The panel says "Wrong ssl certificate". This is so annoying