Plex is not playing 1080p videos anymore

Hello,

My setup is iMac late 2013 -> AppleTV 4

For some time now Plex stopped playing 1080p videos, all I see is spinning wheel.
For the 720p videos it's like playing a bit and loading, playing a bit and loading again.

And it wasn't like this month or two ago.

Is there any solution to this issue?

Thanks!