See screenshot.
This happens every time the page loads on some (but not all) projects. It's impossible to list or create versions in these projects.
Seems like the plugin makes a request to some Heroku app which fails: https://version-manager-cloud.herokuapp.com/versions?projectId=10289&statuses=unreleased
This was created by me, unsure why Bitbucket refers to me as a “former user” there.
This bug basically renders the plugin unusable.