Is anyone else having a problem with Android devices and email attachments?

Once our hosting provider upgraded from 6.x to 7.1 we are now having a problem with email attachments. What happens is that you go to download the attachment and it says "fetching" or shows the progress bar for downloading for a second or two then the bar or message disappears and nothing happens. No download, no error. Try again and you get the same. It is very random - some attachments will download fine. No correlation between file type - sometimes a PDF/JPG etc is fine, sometimes not. No correlation between internal/external, Zimbra/non-Zimbra sender. We've had the exact same attachment from the same person not download on the phone one day but if we resend the next day it's fine. We have around 30 Android phones - HTC, Motorola, Samsung - all have the problem.

We never had any problems with attachments before version 7. Anyone else having issues? Any ideas?