OHhhhhhhh, I think I know what's happening.

Ok, so once you are delegating 0, you can't delegate 0 again. Delegating 0 is undelegating, and if you're delegating 0 again, it's simply not possible to undelegate if you don't delegate already.

So the timeline I think is this:

You delegated XXX amount
You undelegated all to 0
The delegation was coming back, but it still said -XXX in your account (it takes 7 days to return to your funds)
You tried to delegate to 0 again. It said the first error.
You delegated 1
You saw that it's still -XXX because it's coming back. And now that amount has increased by 1.

I think right now you're delegating 1. Delegate 0 again and it will work. And then, you will be delegating 0, but you will still see a -XXX amount in your wallet because it takes time.

Use steemworld.org in the Delegations tab and find how much you're giving. You can remove delegations from there as well.