So I am back again with some further info regarding this problem. The fix with replacing the Server App Video Station was not quite satisfying. Since Videostation 1.4-0643 (Beta) had problems with the Indexing engine. I was able to stream again but VideoStation was not able to index my complete Library. This was due to some bugs within the 1.4-0643 (Beta). So I was forced to switch back to the latest version, since a rollback to older version was not possible.
So after some further testing and analysis I was able to point out the error. I can also replicate the error. The point is, that the VideoStation App (Smartphone App) is buggy since the last update. It is no longer possible to stream on high quality settings over Chromecast. You must set the quality within the App on automatic and it will stream again without the exclamation mark error popping up. Sporadically it is possible that you need to restart your Chromecast dongle (use the Smartphone App for restart). To play any video I must restart also the ds video app on my smartphone. If you change the playback quality you need to restart the app as well, otherwhise you will see the exclamation mark every time you start top stream anything.
So at the moment I am able to stream from ds video over Chromecast, but just with auto-quality (which is pretty poor compared to the best quality, which I am used to). For every video file I'd like to stream I need to restart my smartphone app. If this is not goint to work, restart chromecast as described as well or try to factory reset.
To conclude this is for sure a Bug within the latest ds video update (smartphone app), as it worked before on older versions, also with the FFMPEG with DTS package, which is still working perfectly, despite the buggy app.
What I am going to try ist to find the next older App version of DSvideo and try with that. In the meanwhile I opened a Support-Ticket at Synology Support reporting that issue and also wrote a review on google play store pointing out the latest update is buggy and crippled my almost perfect user experience... Hopefully they will react faster to fix this issue.
Best Regards,
Sonny Rasca