I'm fuming. The hoster of this site, a company I will not mention here, suddenly and without warning decided to block XMLRPC posts, thereby effectively blocking my posts. Well, some of them, anyhow. Now, I don't know why they did it, but they did it stupidly. Thanks a million to Evan for the pointer on the cheap workaround to the HTTP "410 Gone" error I was receiving.
I did contact support, but as is so frequent these days, no answer. Yet.
Update: Hoster support contacted me by e-mail at 11:30 on Monday; their explanation:
das musste vorÃÂÃÂ¼bergehend aus SicherheitsgrÃÂÃÂ¼nden gesperrt werden, dies wurde nun jedoch wieder aufgehoben.