"Mark as Unread" keyboard shortcut in Mail is no longer working and does a completely different action instead.

Hi guys,

I get a lot of spam and rubbishy emails, most of which not worth reading. I ususally just select all and press "cmd+shift+U" to mark as all read or unread. Suddenly I noticed that this shortcut no longer works for me and although it's a really minor problem, it's kind of annoying and I'd love to fix it.

Basically, when I press "cmd+shift+u" now, it doesn't mark as read - instead it brings up a finder window with the option to "open" files (the same window that would appear when you click the paperclip in a new email to browse for attachments), and I have no idea why!

In Mail the shortcut should be the same, it says it's the same shortcut in the drop down menu, and I haven't changed anything. I don't even know what the finder window that appears is for, if I actually use it to open a file then the window disappears but nothing actually opens or happens. What's weird is that when you use the "mark as unread" shortcut and the finder window appears, it doesn't actually become the selected window - mail is still selected. And as a result it means that if you keep pressing the "mark as unread" shortcut, it will continuously open these finder 'file browser' windows... which I've never seen before.

Anyone got a solution?