I have bruteforced the problem and tried about everything to fix it.
This is how I solved the problem for me and my friends: 1. Install software that limits your broadband i.e Netlimiter.
2. Limit the downloadspeed of GTA V to somewhere between 400 and 500 KB/s.
Why (I think) this works:
I think the launcher can't handle full downloadspeed and therefor it discards alot of data, resulting in an error. Think of it as as packages beeing sent down a conveyer and at the end someone is counting the packages. If you send too many, at some point that person will lose the count and have to start over; and you get error 203. But if you limit the speed, that person will manage to count every package and everything will work.
You still might experience some problems now and then, but not nearly as often.