I'm sorry to hear that you had this problem with our installer. It does indeed sound similar to #404, so I'll check our version of NSIS. We're doing some investigation of alternative installers for Npcap, so we may end up using something different prior to the next Nmap release, but at minimum we will fix this for the next release.
@pkExec Yes, unfortunately we have not yet issued an Nmap release with the fix included. To avoid this issue with the Nmap 7.70 installer, you can run it from a CMD.EXE window with the /REGISTERPATH=NO option, or uncheck the "Register Nmap path" checkbox in the graphical installer.