Some emails are blank!

This has just started today. When I get my list of emails in my Inbox (BT Yahoo web mail) the occasional message is totally blank when I click it open. After some fiddling I discovered that if the offending message is moved to a sub-folder, I can open it OK.
Any ideas? One of them was from this forum telling me I had a reply. Another opened first time, so it's not the senders' problem.