I noticed more users having problems editing the settings_general.plist and TargetConfig.plist files. I was able to edit them using iFunbox but not any more.
Since the working situation I updated iTunes (11.0.1) and Garmin Streepilot (2.3) to the latest versions.

This is what I read on the iFunbox site:
Some findings regarding recent bug reports:
1. iOS 6 prevents any 3rd party software having write access to the folder “[app_name].app” in all App sandbox from PC/Mac. Other folders like “Documents” and “Library” are still open for write access. No way to fix it shortly.

Who can still edit the files?