I just ran through RM9’s procedure of File => Export => Dropbox. It worked just fine to make an .rmgc file that I immediately loaded on my iPhone and viewed successfully with the RM mobile app. An iPhone is not an iPad, but I have never had any trouble with the RM app on my iPad, either. My desktop computer is still on Windows 10, in case that makes in difference. My RM is 9.1.3.0
I can’t picture what might be causing the problem on your computer. For sure, I would try rebooting my computer and trying again immediately after the reboot. It’s hard to see what other app might already be using the socket.