Yeah, but our cert is valid ! Restart your computer if it still don’t work to empty any cached ssl cert
Your Certificate is not the Problem.
The Problem is, that transmission is needing an CA BUNDLE file for that verification.
And to build this, some prerequisites are needed (e.g Curl) and if they are not available on the system, that file can not be build.
see the first link i have posted.
So a reboot will not help as long as the prerequisites are not present.