
As far as I can tell, any open connection on the machine running Excel causes the problem. ie. It happens when I'm using Excel on a remote machine. It happens when I use Excel on my machine and I have an open UltraVNC viewer window.
Disabling the UltraVNC clipboard transfer doesn't help. It still happens.
If I do it on a remote machine the UltraVNC server locks up for about ten minutes before the message appears, I can't do anything. Very annoying.
I'm not trying to select gigabytes of data, the spreadsheet file is 152kb. Here's an example which causes the problem: http://www.artlum.com/vncbug.xls
If you open that file in Excel 2003, select the whole sheet (by clicking in the square at top-left next to 'A' and '1') then do a 'copy' you should get the message.