If there is an option to accept the certificate anyway, say yes. I've had this for as long as I've had mumble and it's never given me any issues when I accept the certificate.

If i accept i not working i have to do it like 10 times and then it says SSL authentication failed: The validity of the certificate has expired

I've had this issue before, you're going to have to get a new cerfification. There should be an option in your mumble settings to go to a new certification site.

user>appdata>roaming>mumble
delete mumble.sqlite

The SSL certificate for the eu pugchamp mumble server has expired. Nothing you can do except wait for them to fix it. I've messaged erynn.

e: After further inspection, I'm not sure that this is what is causing your problem, given that the certificate has been expired since August. I would try what pont said, generating a new certificate. You can do that by opening Mumble and clicking Configure -> Certificate wizard. Note that you will have to re-authenticate on any servers you have previously registered on.

just a protip: SSL stuff will always run into issues if your system clock isn't synced, so considering your PC was off for a like a week, the CMOS battery may have died in that time, causing the clock to get out of sync

Change your Nickname when joining the server

Worked for me. Basically my team leader had a channel inside the mumble server were i could only join with the same device and the name so i changed my name and it works. tell me if this fixed your issue

Edit : spelling errors