OK, problem solved. I will put here more info, because it can be useful for others...
A52codec was responsible for all that problems with importing AVCHD's to FCPX. This is AUDIO codec. And location for this codec is different than other codecs. Most of the video codec are in Library/QuickTime, but this audio codec is in Library/Audio/Plug-Ins/Components.
Final Cut Pro X need to transcode AVCHD during importing. And everything is fine if your AVCHD contain 2 channels stereo audio tracks. Then standard Apple codec doing job and Final Cut have no problems. The problems begin when you have AVCHD with A52 codec, which is used for more than 2 audio channels. In my case I've had 4 channels and this A52 codec. Final cut couldn't recognize properly this 4 channel, because audio codec told to FCPX that it must ne surround audio (but is not). Then FCPX get crazy and report that stupid error.
After I've deleted this A52 codec, my FCPX finally started importing AVCHD properly.
I've spend 2 days with Console and Logs to find out where is the problem...