I had some weird issues where the pop-up wasn't actually being displayed on the monitor, but it was opening (I think it was actually on 18.104.22.168). We found this out by someone experiencing the problem, and then adding a second monitor for another purpose and the box would show up on the second monitor. We ended up deleting the Excel items in the registry, and when Excel is reopened it rebuilt them and the popup worked, another option would have been to add a second monitor temporarily to move it back to the main monitor.
Cheers for the reply, sounds like it is probably something similar in this instance then.
Will get him to try some more toggling with the dual monitors to see if we can sort it that way. Regarding your alternative solution, can you recall which registry entries you had to delete in order to resolve?