OK, it definitely makes it a lot harder to help you when you invent your own terminology!
As rec head notes, those are called INPUTS on the receiver. You assign the sources coming in to a specific "input name" like DVR, DVD, VCR, TV/CBL, etc.
My assumption is that your Fios box is assigned to the TV/CBL input. What is happening is a result of the firmware feature upgrade and HDMI Control. When you turn on HDMI Control, it locks out the TV/CBL input from having an external HDMI source assigned to that name. This is actually noted on page 6 of the supplemental manual for the 3808/4308 that Denon published after releasing the Dyn EQ/Vol and HDMI Control upgrade:
The point is that when HDMI Control is on, it "expects" you to have a "Control monitor" linked up with the receiver, and assumes you will be piping back audio from the TV via an optical cable. If you continue to page 7 it notes that for the digital assignment, it locks in Optical3 to the TV/CBL input. So, essentially, that TV/CBL input name becomes "dedicated" to an expected digital audio pipeline back from the TV. The same thing happens on newer Denons when you turn on HDMI Control, but they also allow you to pipe the audio back via HDMI ARC (audio return channel) through the single monitor connection.
I attached the manual supplement here for your reference, that footnote is on the bottom right of page 6 under "Assign":
Denon AVR-4308_3808CI-OM-Addendum-E_008B.pdf 2918k .pdf file