I am starting a new topic, but it may be related to this: http://avidemux.org/smif/index.php?topic=16546.0 (http://avidemux.org/smif/index.php?topic=16546.0)
On Windows x64, HEVC / H265 / x265 support was fixed in Windows x64 nightly 2.6.911 160109. Avidemux can successfully open / playback a HEVC video.
But I'm unable to cut a HEVC video. After setting A/B markers, I get an error Cannot Open Muxer. See attached screen shot.
The version bundled with avidemux does not support h265
Not sure if there is an update
Try with MP4 Muxer instead of MP4v2 Muxer.
Did work here on Ubuntu
Indeed it works with MP4 Muxer on Windows x64 nightly 2.6.911 160109