What's different?

[Sir Osis of Liver]

This forum is blocking my proxy tonight.  Has anything changed?

[Antechinus]

They just don't love you any more.

[LiroyvH]

That seems to be a question that answers itself...
If you suddenly can't visit this site anymore using a proxy that previously worked just fine for you: evidently something must've changed.

Whether it's on our side or on the proxy side is another question though.
With open public proxies, and with TOR exit nodes, it's not uncommon for some of them to be (temporarily) blocked once in a while if they're making too many requests to our servers within a given timeframe. Remember that such proxies are also popular among spammers and kidz, so sometimes they end up in a blackhole.

When that happens: use another proxy, or re-establish connection with the TOR network in the hopes to get a different exit node.
Or use a private proxy/VPN to eliminate such problems. ... or simply use your normal connection, not much of a point in using a proxy here imho.

[Sir Osis of Liver]

Working fine today.  Must have been a series of bad nodes.  Unusual, because any node is only used for a few minutes, then it changes.  Some sites are actively banning Tor, so thought you might be doing that here.

They just don't love you any more.