Well after deleting 'Perian' (wondering that I still have this on my machine) and digging deeper into the Console-App, I've found another extension that could not be started by Compressor (iGetter). So I also trashed that. No success.
I reinstalled FCPX. No success. Reinstalled Compressor. No Success.
Console gave me this info:
19.10.15 23:33:42,569 com.apple.xpc.launchd[1]: (com.apple.xpc.launchd.user.506.100006.Aqua) Could not import service from caller: caller = otherbsd.375, service = com.apple.CompressorHelper-2, error = 119: Service is disabled
19.10.15 23:33:42,570 otherbsd[375]: Could not submit LoginItem job com.apple.CompressorHelper-2: 119: Service is disabled
19.10.15 23:34:52,400 Compressor[1438]: The function ‘CGContextErase’ is obsolete and will be removed in an upcoming update. Unfortunately, this application, or a library it uses, is using this obsolete function, and is thereby contributing to an overall degradation of system performance.
19.10.15 23:34:52,456 Compressor[1438]: -[CQApp launchSharingHelperApp] Cleaning up CompressorHelper since Compressor.app hasn't been shut down cleanly
19.10.15 23:35:30,160 Final Cut Pro[1457]: objc[1457]: Class PMR is implemented in both /Applications/Final Cut Pro.app/Contents/PlugIns/Compressor/CompressorKit.bundle/Contents/Frameworks/AE Core.framework/Versions/A/AECore and /System/Library/PrivateFrameworks/PhotoLibraryPrivate.framework/Versions/A/Fram eworks/PMR.framework/Versions/A/PMR. One of the two will be used. Which one is undefined.
19.10.15 23:38:42,836 Compressor[1438]: CMediaServerMediaDecoder::coreBegin: terminating the app because of uncaught exception: Timed out, server application never registered to receive message
19.10.15 23:38:42,858 com.apple.xpc.launchd[1]: (com.apple.Compressor.504132[1438]) Service exited with abnormal code: 1
19.10.15 23:46:39,588 Compressor[3020]: -[CQApp launchSharingHelperApp] Cleaning up CompressorHelper since Compressor.app hasn't been shut down cleanly
19.10.15 23:46:39,880 Compressor[3020]: WARNING: The Gestalt selector gestaltSystemVersion is returning 10.9.5 instead of 10.10.5. Use NSProcessInfo's operatingSystemVersion property to get correct system version number.
I also started Etre-Check, which gave me this (confusing) info:
[running] com.apple.CompressorHelper-2 - com.apple.CompressorHelper-2: Executable not found!
Maybe there is some developer around who can use this to further investigate. That would be charming. Thanks!