Things seem to point to the host - and it's ability to post outbound to QBMS. Downloading latest is good - as the 1.5 version did not use the correct URL for desktop mode. Are you using cURL? (you should not have to). You didn't mention where your site is hosted. Also - when exactly is that error coming up?

Hard to say without seeing some log data - but symptoms favor this being at the host level, and possibly some change that occured there. As you say - you made no changes to the site, and others are obviously processing with QBMS ok.