Attachments can't be clicked to open

Hello,

Sometimes when I read mail with attachments, I see the paperclip icon and the number of files attached, but only a semicolon where the filenames should be. This is intermittent, as the same attachment filenames might show up another time. However, I can't force them to appear. This is very annoying as most of the mail I receive has attachments which I need to be able to open. I don't think the fixes already suggested regarding antivirus settings would apply, since the behavior comes and goes with nothing being tweaked by me. Thanks for your help.