I have been having the same problem for about a week or two now, only on MKV video (AC3 audio). Tried changing audio to PCM without success. Tried to reinstall videostream plugin without success.
Windows 10 x64 / Chromecast 2 / Videostream premium

I have the same problem but i only cast directly from PC and when i disconnect from internet it does not work.

Actually there's an iOS app too out there, so you're sorted in either case as long as you own a smartphone.

When is this going to be fixed? MKV with DVD Subtitles (spu) are still not working. (I'm also on premium account) I have all my dvd:s ripped on my computer with these subtitles and I can't watch them with this.