Hello @Michelist 
There is a problem when rendering does not start at frame-0.
This has been reported on the forum:
Render (not a very informative title)
Animation export bug in latest release
There is a formal bug report:
459993 – Frames being named incorrectly and program being unable to found specified frames to being encoded
In that bug report, the reply to my question was ‘interesting’ but I didn’t try it.
I have created a animation that shows this problem.
It’s to do with frame counter management - it doesn’t count properly.
I’ve no idea when it will be fixed.
All I can suggest is that you render out from frame-0 then use a video editor for the final cut.
The alternative is to render out from frame-0 to an image frame sequence then Import just the frames you want then Render those out again. It’s not good but it would work.
Edit:Add: Actually, if you Render to an Image Sequence then it does seem to be ok for rendering starting in the middle of the animation.