If I cut at I-frame, sometimes I get black frames, until the next I-frame (in copy mode). It only affects the first cut for some reason.
If the latest nightly (http://avidemux.org/nightly/) exhibits the same issue, please provide a sample source video and exact stept to reproduce.
Alright, tested avidemux_r170727_win64Qt5_452 and the cut is ok ! Tested on several other files that gave me black frames and all worked.