I've been using TP forever, but this is the first time I've tried to run an external tool. If I configure the following tool in preferences, it works correctly:
It does not appear to be handling the embedded space in the command or directory paths correctly in spite of the double quotes. Error 267 is ERROR_DIRECTORY: The directory name is invalid.
TextPad expects to find plumb.exe in the system subfolder of its installation folder. It is placed there by its installation program, so you could try reinstalling TextPad.
Please also check that you have not accidentally created a copy of textpad.exe on your desktop, rather than a shortcut link.
I'm pretty sure this is a bug. I searched the forums before posting and saw that advice repeatedly, but it does not apply.
1) the command works from preferences, which means it could find plumb.exe in that context. The error only occurs with the "Run..." dialog.
2) I did verify that I'm running TP from a properly configured shortcut and not a copy of the EXE
3) I also verified that plumb.exe is in "C:\Program Files (x86)\TextPad 7\system\plumb.exe"