Well, thanks Ken, but I don't know what to say,
I'm completely puzzled by all this, see attached.
There is no option for 720p50 in my Easy SetUp,
but somehow my Sequence Settings (Picture 2) show "Compressor: DVCPOR HD 720p50 (same like my clips),
but again, have a look at Editing Timebase, it's 25. I tried to change it in Advanced pop up menu where there is an option of 50 but I can't change it, I'm getting Warning "Can't change it, do it in Editing Timebase", but I don't have 50 fps option in Editing Timebase pop up menu.