Comtrol ES8510-XT - Manual (Page 137)

RocketLinx ES8510 and ES8510-XT User Guide: 2000513 Rev. H VTY Configuration Locked (Error Message) - 137
Configuration Using the Command Line Interface (CLI)
VTY Configuration Locked (Error Message)
An alert message (VTY configuration is locked by another VTY)
appears when multiple users are attempting to configure the
ES8510. If the administrator is in Configuration mode, then the
web users cannot change settings. The ES8510 allows only one
administrator to configure the switch at a time.