Little update. I tried a lot with the 5.4 Kernel. I tested Bionic and Buster, but both got the problem above. Then I made a post in the armbian forum and one guy told me to switch back to the 4.4 Kernel and look there, it is working!
The problem of the 4.4 Kernel was for me, that plex videos in a docker container want play longer than 5 minutes with direct play. The best solution was to switch over to embyn, that solved my subtitle problem too