Lots of developers tie into events (like when you change selection or change a
cell) and have their code do something "important".  

So that developer could be explicitly trying to stop you from pasting, but more
normally, lots of macros will clear the cutcopymode in excel.  And you're just
suffering from that problem.

The solution is to disable all macros -- like you did.

Or temporarily disable event monitoring.

To try this:
hit alt-f8 (to get to the VBE)
hit ctrl-g (to see the immediate window)
type this and hit enter:
application.enableevents = false

When you're ready to go back to "normal", do the same thing, but change it to
true.