Bitbucket SSH Repository Mirroring

Hi, I want to synchronize our gitlab repositories with bitbucket. Therefore I wanted to set up a repository mirror over SSH. I created the mirror and added the public key in Bitbucket.
But I can’t get Gitlab to synchronize the repo with Bitbucket, I get the following error

I’m running Gitlab CE self-managed with docker (current gitlab version: 12.9.2).

I have already tried the mirror over HTTPS, but there I always get a “Fatal authentication failed” error, which can’t be because I used my own account.
I have also already added the following SSH settings in the config… without success

Host *
StrictHostKeyChecking no