First of all BIG thanks Jim for your time you put in your answer and trying to help me out!
I was already afraid that there will be no easy sorting out of the user-home folder conflict with its contents and what in particular are causing the crashing of Motion.
by the way: Motion launches okay and shows the "Project Browser" window for a couple of seconds and then crashes and is not usable at all.
also the Console crash-report file from the DiagnosticReports shows basically the same information then the Problem Report ... unfortunately no traces in there for more specific possibilities what to look for in the home-directory.
basically I delete the preference file before each try and have checked the fonts and all are ok, permissions are fixed (although I am not big fan of letting them repair: generally causing more problems then solutions), cache files cleaned, deleted all Motion and FCP add-ons (oh i am desperate) and not sure how to identify legacy FCP studio programs and where to look for them ... same as for audio-folders other then ~/Music/
well no culprits identified so far.
/Library/Application Support/ProApps is another folder I am not sure what kind of content it should include. I "cleaned" it already a while ago down to:
/Library/Application Support/ProApps/Internal Plug-Ins/BrowserKit/FileSystem.bkplugin
/Library/Application Support/ProApps/Internal Plug-Ins/BrowserKit/iLife.bkplugin
I guess the only solution is as you recommended going folder by folder, file by file ... omg!
but honestly: I don't think i'll find the time to do this.
what I am still curious to find out is: why was Motion 5.0.4 working flawlessly and since the update to 5.0.5 it became unusable? also downgrading didn't work for me? what did happen during the update? what was changed and/or modified to get Motion unusable? all other apps are working perfectly (including FCPX) so there can't be a serious conflict in the user-folder?