I am using an outdated browser; but still - this pop-up message hadn't started until just within the last two weeks. Has some change been made in that time?
You are apparently using a terribly outdated browser that doesn't implement
Server Name Indication:
$ openssl s_client -connect forum.eurofurence.org:443
[..]
Certificate chain
0 s:/C=XX/ST=There is no such thing outside US/L=Everywhere/O=OCOSA/OU=Office for Complication of Otherwise Simple Affairs/CN=dapper.tigress.com/emailAddress=root@dapper.tigress.com
i:/C=XX/ST=There is no such thing outside US/L=Everywhere/O=OCOSA/OU=Office for Complication of Otherwise Simple Affairs/CN=dapper.tigress.com/emailAddress=root@dapper.tigress.com
[..]
Verify return code: 10 (certificate has expired)
$ openssl s_client -connect forum.eurofurence.org:443 -servername forum.eurofurence.org
[..]
Certificate chain
0 s:/C=DE/CN=forum.eurofurence.org/emailAddress=dohmaihns@yatho.de
i:/C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing/CN=StartCom Class 1 Primary Intermediate Server CA
1 s:/C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing/CN=StartCom Class 1 Primary Intermediate Server CA
i:/C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing/CN=StartCom Certification Authority
[..]
Start Time: 1432735217
Timeout : 300 (sec)
Verify return code: 0 (ok)
$
Do yourself and the Internet a favor and update to a current browser version, yours likely has gaping security holes.