Thanks for the help.
Using a new user let me connect to the diskstation.
Cheers, learning all the time!
Now what?!
Three files which I think could be the problem.
1. com.apple.finder.plist in your preferences folder.
2. com.apple.sidebarlists.plist also in your preferences folder.
3. .DS_store on the Desktop, this is an invisible file.
Now, start with the first one, drag it to the desktop, log out and in again, problem gone you found it, be aware if you move this file all finder preferences are reset.
If this was not the problem move the second file to the desktop, log out and in again, problem gone it;s this file, also here, sideBar preferences are reset.
Third option is the invisible file, move it to the trash, log out and in again, same procedure.