Can't play .mkv files on my Samsung UE40D7090 from synology

Hi all, I'm new to Serviio and just managed to install it on my synology DS716+II . There is currently the synology mediaserver running, too. After indexing all my content it appears that I can't play .mkv files from Serviio to my Samsung TV no matter of the size. But there's no problem with the mediaserver of sysnology's. Could you please give me a hint what I can do? Thanks in advance for your feedback.