I figured it out 
I found this post: https://forums.plex.tv/t/synology-ds1019-hardware-transcoding/384853 I hadn’t seen this one before…
On the user-1 account I had subtitles enabled, on the user-admin account not. That was the only difference.
It appears that if I either disable the subtitles on the video and/or turn of hardware acceleration the problem is resolved. That is unfortunate, because I do want to be able to use subtitles and I would have prefered to leave hardware acceleration enabled.
But at least it works now 
If anyone can think of a way that I can have both, please let me know! Well, without buying a new host machine 
