TF255356: The following error occurred when configuring the Team
Foundation databases: Error occurred while validating servicing step
Add Running User to the TFSADMINROLE for component Install during
Install: Unable to deserialize servicing step data to type
Microsoft.TeamFoundation.Server.Servicing.TFCollection.AddSqlRoleMemberStepData..
For more information, see the configuration log.
Error occurred while validating servicing step Add Running User to the
TFSADMINROLE for component Install during Install: Unable to
deserialize servicing step data to type
Microsoft.TeamFoundation.Server.Servicing.TFCollection.AddSqlRoleMemberStepData.
This is a known problem registered in Microsoft Connect:
So my guess is that somewhere in your domain, group, service or user name used during installation and configuration of TFS, there is a symbol included that cannot be deserialized correctly. Thinking about symbols like < > & %.