Several of my users (including me) have had problems connecting to our Bitbucket server after upgrading to Sourcetree 2.5.5.0.

They are getting errors in the Sourcetree log file as shown below

ERROR [2018-04-27 10:26:04,648] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Insufficient authentication credentials
ERROR [2018-04-27 10:26:04,895] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Failed to list repositories for user [joebloggs]
ERROR [2018-04-27 10:26:04,909] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Insufficient authentication credentials
ERROR [2018-04-27 10:26:04,917] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Failed to list teams for user [joebloggs]
ERROR [2018-04-27 10:26:04,922] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Unable to read memberships for the user from the remote host
ERROR [2018-04-27 10:26:08,570] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Insufficient authentication credentials
ERROR [2018-04-27 10:26:08,576] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Failed to check login for user [joebloggs]
ERROR [2018-04-27 10:26:10,437] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Insufficient authentication credentials
ERROR [2018-04-27 10:26:10,443] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Failed to list repositories for user [joebloggs]
ERROR [2018-04-27 10:26:10,450] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Insufficient authentication credentials
ERROR [2018-04-27 10:26:10,457] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Failed to list teams for user [joebloggs]
ERROR [2018-04-27 10:26:10,461] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Unable to read memberships for the user from the remote host
ERROR [2018-04-27 10:26:13,799] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Insufficient authentication credentials
ERROR [2018-04-27 10:26:13,804] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Failed to check login for user [joebloggs]
ERROR [2018-04-27 10:26:16,380] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Insufficient authentication credentials
ERROR [2018-04-27 10:26:16,389] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Failed to list repositories for user [joebloggs]
ERROR [2018-04-27 10:26:16,395] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Insufficient authentication credentials
ERROR [2018-04-27 10:26:16,399] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Failed to list teams for user [joebloggs]
ERROR [2018-04-27 10:26:16,404] [1] [SourceTree.Exceptions.RemoteHostException] [.ctor] - Unable to read memberships for the user from the remote host

If we remove the user in Sourcetree

Tools - Options - Authentication

by clicking on the username in Accounts and delete this and then add it again it works fine.