I had the same problem. Unfortunately I do not believe there is a cure, just a work around.
1. There was a suspicion that it had to do with the ftdi or lack of an ftdi chip set. I had the failure with both I believe . But I might be remembering it wrong. If you are using a non ftdi usb-serial adapter then get one with the ftdi chipset. If you are an iexos it already has it in there, so look at the next point.
2. What worked for me was to change the pc being used to load the code. Not ideal I know. I couldn’t get it to work using my windows 10 tablet. It did work in a bootcamp Mac air, and it works fine on my Dell laptop.
Sorry man, that’s all I can tell you. Find another computer.
Wes, Southport NC
EXos2-GT PMC-8, iExos 100
ES ED 127, 10" LX200GPS+wedge, Astro-Tech 8" Newt, ETX-90, 60mm no-name guide scope ~ 260mm FL
Polemaster, Orion ST-80 and SAG, ZWO 290MM, D5300 astro modified
Nina, Bootcamped Mac Mini control computer, RDP to iMAC
110 amp hour lead acid deep discharge battery for field power
Electrical Engineer, Retired