See the GitLab article:
Webhooks and insecure internal web services

Add the domain of the source server to the allowlist on the destination server. This resolved the error for me. However, upon completing the import “successfully” I got “No Repository.” So the only way I was able to import it in my case was to fork it on the source (not necessary if you can access the repo settings), export (to a tar.gz file by default), then import the “GitLab export.” More info on this in their “ Migrating projects to a GitLab instance ” article. You could then setup Repository Mirroring , but this is a paid feature unfortunately.