we're having this problem as well. any new info?
Everything seems to work after the upgrade to 3.5U2. I don't know if the upgrade solved the problem or simply going through the reboot process did.
ours ended up being another vm using the serial port. obviously both cant use at the same time.
That was the first thing I checked. It was rather odd, as the problem was localized to a single server. I'm not convinced that it was software related, I half believe that it was hardware related.
I've just had a client call me with the same issue. I got them to check if they had another VM's using the serial port and they had. Usual thing, "first in, best dressed". Disconnected the unwanted serail ports, all now working. Thanks:-)