Ok, I will try to create some logs. However, @Kontrarian might be in to something here: if I follow the instructions (enable logs + two reboots) the frame drops seem to be difficult to reproduce. When the unit has been on for some time and something has been watched they get more common. I will keep the logs on nevertheless and try to create new logs when I see frame drops.
By the way, my projector only supports: 480p, 576p, 720/60p, 720/50p, 1080/60p, 1080/50p, 1080/60i, 1080/50i, 1080/24p. I have whitelisted those as sometimes the projector just showed a blue screen when Kodi tried to set an unsupported video mode. However, without “Sync playback to display”, how can Kodi play video flawlessly if the frame rate of the video does not match the one used in the display device?
This is what Adjust Refresh Rate is typically used for.
Just the reference for ‘Adjust display refresh rate’ and ’ Sync playback to display’ to better understand the meaning and difference:
https://kodi.wiki/view/Settings/Player/Videos#Adjust_display_refresh_rate
Sure, but as I said my projector does not accept all possible refresh rates and I have whitelisted the accepted modes. So I assume “sync playback to display” should be enabled on my setup to playback also the media which has frame rate not compatible with my projector.
This functionality doesn’t exist on Vero with hardware acceleration.
Some logs showing what your display is capable of would help.
I think you will find it does, except for 25 and 30Hz which Vero will play at 50 and 60Hz respectively. I’m not aware of any display other than maybe PC monitors that won’t sync to the ‘fractional’ framerates 23.98Hz and 59.94Hz if they support 24 and 60Hz. So if you haven’t whitelisted those fractional rates (which you have not, based on the log you posted) that would cause issues.
The reason why I had set up the whitelist is that I have some videos in 720p25:
Video: h264 (High), yuv420p(progressive), 1280x720 [SAR 1:1 DAR 16:9], 25 fps, 25 tbr, 1k tbn (default)
And without the whitelist those won’t work with OSMC on my projector (blue screen). I removed now the whitelist and I will see if it helps on the frame drop issue and if all other media plays ok. However can I somehow “blacklist” 720p25 to get this files to play as well? 
Don’t get rid of the whitelist. Just add the fractional rates 23.98 and 59.94 and tick the option to use double refreshrates. Without a whitelist those 1280x720p25 vids will be playing at your GUI resolution which is 1080p24 and conversion from 25 to 24 Hz will be straining the SoC.
That makes sense, thank you, I will give it a shot!
The fractional rates might also explain the occasional frame drops - we’ll see if this help.
I believe this worked, as I have not seen now dropped frames!
However, now I ran into a media that the projector cannot playback. VLC tells that the media is H264, 1080P, 29,970628. Projector display stays blue but sound plays from the receiver. With VLC the video plays without problems. I removed 1080p / 23.98 (2D) from the whitelist but even that did not work.
Is there any trick to somehow play this file, even with some dropped frames, without messing with the whitelist? Any way to find out without starting the playback what video mode OSMC will pick for a specific file?
Hard to say without debug logs.
No.