So I got the same problem occuring when trying to install the game with Gameforge Client.
I always tried to install it on E:\ (where I install all my games and softwares, ..., C:\ is mostly for my system) and with different solutions :
Change the region, disable shortcut creation, enable all privileges for admin/user on my PC on the gameforge client executable, use simple folder names such as "fafa", but nothing changed....
Then I tried to install it on the default C:\ disk as suggested by a few people here, in a random "ff" folder at the root of the disk (could probably be a different folder name and not placed at the root).
Magic! It worked and now the game is downloading.
So, whatever with the D:\ Disk, I will let it this way.
Yes, this a launcher problem. I tried to do the same with other games and it gives the error too. The problem is, like other people here, I dont have enough space on the main drive and selecting another drive just crashes the client.
I installed gameforge client today just for this, a friend of mine had the launcher installed before and he was able to change the directory. Dont know how to relate this but there it is.
Still no solution.