"server certificate verification failed"

Hi, I want to install Avidemux on an Ascii Devuan, so I added in
sources.list:
deb https://deb-multimedia.org/ stretch main non-free

But when I launch "aptitude update", I get:
"Err https://deb-multimedia.org stretch Release
server certificate verification failed. CAfile:
/etc/ssl/certs/ca-certificates.crt CRLfile: none"

How do I solve this?
--
Cordialement, Stephane Ascoet