I've opened a support ticket regarding this - but I wanted to see if anyone else was experiencing the same problem (and if so found a work around).
Since our 6.0.5 to 6.0.6 update this weekend, many of our users are having problems opening attachments (specifically messages forwarded as attachments from other systems). The new window opens but hangs at 'loading', as shown in the screenshot below. The same thing happens for some people when they try to open any message in a new window (I had to have people uncheck 'double click opens message in new window' in their settings as a workaround).
Any suggestions or workarounds that anyone can provide would be very much appreciated.