I had exactly the same issue with my unit - a simple fix, hopefully yours will be the same.
My unit does not have TMC - yours, I suspect, is the same (not available on the Win CE units with newer menus). In my case, I looked at the Primo Sys.txt file and found a few entries relating to TMC. As my unit doesn't support this, I deleted all entries (make a backup first, just in case) and since than, all has been well. Didn't have to change Baud rates or anything - it was the TMC references that were causing the issue.
Hope this helps,
Cliff