The problem originates with [https://github.com/notepad-plus-plus/wingup/issues/9|this bug] in Notepad++’s updater.

I’ve circumvented the problem for now by escaping the full download URLs. It works in the debug version, but we'll have to wait for the next release of Notepad++ to see if it really works in the final version as well. I’ll close this bug at that time (if I don’t forget).