During our net this evening, my 6400M had a message come on the screen saying that the radio was in use, try again later. It rebooted and came back to where I was. Am running the latest software. no third party stuff.
Based on what you have described (and you aren't alone, I have seen this report elsewhere), I think what is happening is that the client is getting disconnected from the radio (server). At this point, the client attempts to reconnect as it sees the radio online. However, the radio hasn't timed out the original client connection yet and reports that the radio is "in use."
tl;dr: This is a software issue that we will address. Currently tracked as #6143.