bitbucket.org has intentionally disabled ssh multiplexing, so edit your ~.ssh/config file so ssh doesn't try to multiplex when connecting to it.
If you have ControlMaster at the top level of config, make sure it is removed as it supersedes any Host section configuration.
Then add:
Host bitbucket.??? # .org and .com
ControlMaster no
Host *
ControlMaster auto
ControlPersist yes
ControlPath ~/.ssh/socket-%r@%h:%p
Note that Host bitbucket.org must come before Host *.
Also, the ControlPath should only be writable by yourself.
Then, if the ControlSocket named in the error message is still there, delete it. (It will auto-delete based upon the timeout in ControlPersist.