I've had this problem, probably since installing version 4.0x, but it's gotten annoying enough to ask for help with it.
I do a "copy" or "cut" on one PC, move the mouse over to the other PC in seamless mode, and then "paste". Nothing happens, or worse, the 'paste' causes the browser window to hang.
All other functions work great!
Oh, an update: I was having problems with Multiplicity on my PCs, which were being blamed on Webroot. The issue turned out to be the Intel Ethernet Connect (7) I219-LM chip/firmware in my Dell OptiPlex 7060 PCs. I bought a couple of cheap TP-Link Ethernet cards and installed them, and Multiplicity has worked great ever since, with or without Webroot. Thanks to those who pointed me in that direction.