Are you over the user limit, or are all accounts you create unable to login at this point? Had a similar issue, but possibly not the same.
After installing i set a admin username/password, but could never login. I attempted "forgotten password", set a new password but still could not login. Due to your one-line error description, it's hard to judge if we were seeing the same issue. Please elaborate.
My problem was solved via:
We’ve been building a plugin to integrate Bitbucket Server and Jenkins CI, and I’m excited to announce that our alpha is ready to download and install. It lets you seamlessly configure a Jenkins job ...
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