The feedback indicator appears when you create a "send" from a mixbus, and send it back to an input strip, which has the potential of creating a feedback loop. In this case, Mixbus cannot calculate a valid "latency compensation value" for the mixer engine, and it will not process audio.
If a session is saved and then re-loaded in this state, you should see the red "feedback" light blinking at the top of the editor when you load this session, and you won't hear any audio when you try to play the session.
To solve this problem, you should remove the "send" from the mixbus back to the offending bus. Mixbus isn't intended to work that way. Instead you should add a reverb plugin on a mixbus, and use the knobs from the channelstrips to send to the reverb.
It is possible to "override" the system and make this work. You can right-click on those Sends, and choose "send options->allow feedback". That's not guaranteed to be supported in all future versions of Mixbus though, because it's breaking the "rules" of our internal mixer latency compensation. But since it only incurs a little extra delay in the reverb/delay sends, you probably won't notice.
Alternatively, you can just create a reverb and delay plugin on the source tracks or a group bus; and then adjust the wet/dry control directly in the strip. You can put the reverb either pre- or post-fader, depending on your needs. See this post for some ideas: