Yesterday I posted a tutorial explaining how to use Gpg4win to verify electrum downloads. It includes the same link as yours (copied from the electrum website). It worked fine yesterday, and today gives this error.
Service Temporarily Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
You can find a copy of ThomasV's key here.
https://github.com/spesmilo/electrum/blob/master/pubkeys/ThomasV.ascYou can also find it at this key server (which is presently online).
https://sks-keyservers.net/pks/lookup?op=get&search=0x2BD5824B7F9470E6If it's any use there's also a copy of ThomasV's key in my tutorial.
https://bitcointalk.org/index.php?topic=1836004.msg18345387#msg18345387