Comment posting and image uploads broken

I’ve got two WP blogs hosted by the same service. One is my sandbox for fiddling with and is still running 2.11 and the other is my real blog and was running 2.12 but due to some fiddling it is at the moment 2.11.

My problems (among others) are that neither blog manages to accept comments nor upload pictures from the writing interface.

I’ve pretty much done what I can think off and can only imagine this is not a WP problem but something to do with the hosting. My main blog hofteigur.net (using Kubrick) will load wp-comments-post.php but stop with no reported error and the browser window blank. The sandbox (using K2) will report no error, but seems like the AJAX comment system is… well “stuck”.

I’m getting a bit tired waiting for support at my hosting service to fix this so I’d be very grateful if anybody can hint at what the problem is.