It appears that the HTTPS certificate for forum.xwiki.org expired.
The trouble with using letsencrypt, is that you need to make sure that it is renewing…
I would have emailed, but with the closure of the email list…
If assistance is required in “taming” letsencrypt, do send me an email.
Thanks for reporting. The sysadmin is currently on holidays but should be back tomorrow. So hopefully a new certificate will be setup tomorrow. Sorry about that.
Certificate issue fixed and discourse has been upgraded to the latest version too!
Sorry for the issue.
And now I can use Chrome instead of IE 
As this certificate expires on the 25th of October, I would suggest that the person responsible manually checks that the certificate has renewed before 24th of October.
Normally there’s a crontab that autorenew. I don’t know why it failed though… so indeed we’ll need to check that. At least now I know how to renew so even if there’s no sysadmin avail, I can do it