This error is caused by the add-on.
For example, if you launch two clients on the same machine (probably if you are using Password Depot on a terminal server) and receive this error, you must assign to users different port numbers in order to correctly communicate with the browser add-on:
To do so, open the program options by going to Edit -> Options -> Browsers and change the value of the WebSockets port. The same value must then also be set in the browser add-on (within the browser itself).
For more information, click here.
If you use a terminal server we also recommend taking into consideration the following article: