By default, GIMP-portable tries to use a folder below the one it has been started from for temp and swap. That’s just fine if you use it on your usb-stick, but it bugs you with an error message if GIMP is supplied from a read-only network drive: “Unable to open a test swap file.”
This can be fixed by editing (or creating) the system-wide gimprc file, which is located in \Data\settings\ …
First, I tried to use the windows-style %APPDATA% variable, but GIMP does not interpret it correctly. Digging a bit in the web revealed the way how to do it, they have to be used in shell-style including curly brackets like this:
(temp-path “${APPDATA}\\gimp-2.6”)
(swap-path “${APPDATA}\\gimp-2.6”)
(undo-size 512M)