There's a forum post on this issue from months ago, and I don't see any resolution to the problem. I tried searching here, but I didn't see the same issue listed in the Bug Reports.
For us, it doesn't seem to matter what the age of the Mac is. We have problems with the Host trying to type within a Mac session, and it seems to come and go randomly within the session. When they try to type, they just get the letter "a" no matter what letters they press on their keyboard. The end user can still type just fine, it's the Host that has the problem while connected to their Mac.
Here's the thread for more information:
Customer support service by UserEcho