I also had to add an exception.
Win10 Firefox.
Cleared the exception, warning comes up.
Clear cookies .. no real change.
Did the log out/log in routine .. same results.
The warning I see tho is not about certificate, FF says 'weak encryption'
Click on the yellow triangle warning where the https green lock should be, and it opens the dialogue box.
It just says, 'the connection is not secure, this page uses weak encryption'