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.

Same problem as with bug #1509909. Might as well write some better system() function that'd just call CreateProcess, but it won't be able to process command line processor directives like "if" and output redirections.