PlanAhead tool is saving the custom Editor path correctly in the planahead.ini file. However, when PlanAhead is launched again, the planahead.ini file gets modified and loses the '\' characters separating directory levels.
The workaround is to add the path to the text editor in the Operation System PATH, and then enter only the executable name and options in the Custom Editor Definition window.
Example: Instead of entering "C:\Windows\notepadd++.exe [file] -n[line number]", change the entry to "notepadd++.exe [file] -n[line number]" and make sure "C:\Windows" is included in the PATH environment variable.
This issue is resolved in PlanAhead tool 13.4.