We have this issue since years, too and I don't belive that it's related to the RealPort.
Because affected are two AnywhereUSB/14, two USB/5 MHC and one old USB5 legacy box.
The USB/5 have no RealPort feature nor such check box in WebUI.
The Operating System on the clients was over years, XP, Win 7 - 8.1 and some Windows Server 2k03 / 2k08.
We updated FW and drivers regulary without any improvement. A Host reboot is mostly required in more than 80%.