Specifying a project name doesn't work - What should I do?

I downloaded UE4 a few days ago and wanted to create my first project, but there is something wrong with my ‘name’-field.
I want to name my project “Firstproject” which only contains alpabetic characters and should work without any problems. Still, the programm always says “Path may not contain the following characters: #”
There is no ‘#’ anywhere in the name so I don’t know what I do wrong. Because of that, I can’t even start to create a project, no matter what I type in the name-space.

I already deinstalled and downloaded UE4 again and restarted my computer, but it still does not work.

What is the exact issue here and how can I finally create a project?

Thank you for any help.