Bug in packhdr with path containing spaces?
The !packhdr command seems to having problems accepting paths with spaces in them. From the manual :
!packhdr "$%TEMP%\exehead.tmp" '"C:\Program Files\UPX\upx.exe" "$%TEMP%\exehead.tmp"'
When you put UPX in a path that contains spaces, NSIS cannot seem to find UPX. Putting UPX in a path not containing spaces does work fine.