The topic and callback are the same.
Yes this is the strange thing, yesterday my test was to unsubscribe all the webhooks I found via https://api.twitch.tv/helix/webhooks/subscriptions
I left the api still working without any webhooks, no events were triggered which basically means that there are no webhooks with this callback any more. Today I tried to subscribe my needed webhooks again and got the above mentioned error message after 9097 webhooks. I actually have no explanation for this.