Unselectable i/o insert

Had a strange issue yesterday that I've never seen before.

New session, only 1 stereo track assigned to "no input" and "analogue 1-2" output on my first 96i/o. When I tried to insert "analogue 3-4 I/o", it told me that it was already in use, and deactivated it. I could not activate it no matter what I tried. It's not a deal breaker, but very strange. Has anybody ever run into this issue?

[Micks_Mix]

Any hidden tracks? Did you create the session from scratch or from a template?

Check you I/O setup, maybe you have some multi output routing etc. (marked with a "+").

I figured it out. My audition path was set to output 3&4.

Thanks for the reply Micks.