It appears you are running into HDCP (High-Bandwidth Digital Copyright Protection). Per an FCC regulation, all set top boxes were updated with this software as of 01/01/16. I'll link a video below that explains this in more detail.
HDMI, Your Slingbox, And You [Video]:
Now, HDCP/SOC have developed quite a bit since this video was posted and some TV Providers will no longer allow Video and/or Audio to be transmitted if there is any HDMI cabling what-so-ever coming out of the set top box. In this scenario, you can invest in an HDMI adapter with HDCP Bypass, shown and discussed in this article:
I hope this helps!
Sling Media Technical Support