Debugging with xdebug: change mapping after linking new sources

I am running a typo3-installation on my local machine which I am trying to debug.

Using a common setup, I linked the typo3-source via symlink.

After updating the typo3-sources, the symlink is changed to the new sources.

In phpstorm, I changed the respective settings:

• in Settings -> Languages & Frameworks -> PHP I deleted the include-path to the old sources and added the new sources
• in Settings -> Languages & Frameworks -> PHP -> Servers I tried my best to add the right mapping, admitting that I still find this totally confusing

If start a debug session, the debugger stops at the fist line with massage "The script '/path/to/new/typo3-source/index.php' is outside the project."

Clicking on "Click to set up path mappings" opens the "Resolve Path Mapping Problem" dialog.

The header of the window says:

• file path on server: '/path/to/new/typo3-source/index.php'
• file path in project: '/path/to/OLD/typo3-source/typo3/index.php'

Obviously the path to the old sources is somewhere in the system.

I controlled all my settings and didn't find a single mention of the old path, I cleared caches via file -> invalidate caches/restart, I restarted the IDE, but to no avail.

Any suggestions how to proceed?