Comtrol ES7528 - Manual (Page 150)

150 - VTY Configuration Locked (Error Message) RocketLinx ES7528 User Guide: 2000509 Rev. C
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
ES7528. If the administrator is in Configuration mode, then the
web users cannot change settings. The ES7528 allows only one
administrator to configure the switch at a time.