Can't Delete Auto-Nav Block1 user found helpful
Try to figure out which permission is needed to edit site-wide blocks. I myself have no idea of which one it is right now.
I had the same thing happen once.
To remove the Auto-Nav block, I went into the matching global area stack and clicked on "Clear Global Area".
Dashboard > Stacks & Blocks > Stacks > View Stacks button > select View Global Areas
I tried both of your suggestions with no joy. The guys over at C5HUB were nice enough to intervene and fix the problem. In case it helps anyone else with this problem here was their solutions:
We logged in to your site and fixed the issue. Looks like there may be a bug in concrete5 where the areas can become out of sync. We have seen this before in 5.6 but not in 5.7 - we thought they had resolved this.
Anyway, we simply added another block to the affected area, published the page and then deleted this and published once more. We then added the autonav back to the site to give you a good starting point.
Can thank you enough C5HUB :-)