OS X 10.6 Mail Won't Use MobileMe Junk Folder

[NightFox]

I've always had Mail set up on my two Macs to store junk email on the MobileMe server, and it's worked fine up until a few days ago, since when I've been getting a warning triangle next to the Junk folder in Mail saying that it could not use the folder on the server, and it created a local "on my Mac" junk folder instead. This also automatically deselected the "store junk email on the server" option in Mail.

I checked MobileMe, and the junk mail folder icon had disappeared. I managed to recreate it by marking an item as junk in MobileMe, but I still keep getting the same error on my Macs: I reselect the option to store junk email on the server, but it fails and reverts to storing it locally. I think I may also be having occasional similar problems with the Trash folder, as I see I've also got a local one of those created.

Anyone had anything similar and can suggest anything?

Thanks