Problem 1. XAMPP doesn't support installation into a folder where the path contains spaces. The Windows installer actually raises an error message towards the end of the installation process. This should be fixed. The components of XAMPP should be fixed to work on Windows without such restrictions.
Problem 2. The installer doesn't warn about this. I can start the installation in a path with spaces and it only fails at the end of installation. I have to uninstall, reboot, reinstall. At least the installer should warn about this right at the beginning. It does warn about User Account Control and not installing into C:\Program Files, so I entered D:\Program Files, then it failed. Why does it warn about some things and not others?
Problem 3. XAMPP doesn't support installation into a folder where the path contains an accented letter. This should be fixed. My Windows username is my given name, Márton, and I didn't even decide this. Windows gave me this username after logging in to Windows with my Microsoft account. So I cannot install into C:\Users\Márton\ProgrammingStuff either.
Problem 4. When I enter C:\Users\Márton\ProgrammingStuff as the installation directory, the message I get is not correct. It says the installation path can only contain letters and certain characters, but accented letters are letters! At least the error message should be correct.
I believe that in 2017 it is increasingly embarrassing for a modern application to 1. not support all possible paths, and 2. to fail in wrong ways instead of raising a correct error message early on.