PlantUML produces error when specifying asynchronous messages

Question

I have this PlantUML code (.puml)

@startuml
title Asynchronous Message Example

participant Alice
participant Bob

Alice -> Bob: Request
Bob --> Alice: Acknowledgment
Alice ::> Bob: Asynchronous Message
Bob ::> Alice: Asynchronous Response

@enduml

and I get this result:

The plantuml version I am using is:

Answer 1

Asynchronous messages shall be encoded as ->> or <<- according to the plantuml online documentation on sequence diagrams and the official language reference manual.

It is therefore normal that your plantuml instance complains about the use of ::>, as it is not the correct syntax, independently of your plugin.

How to solve it: 1) Check with the author of the puml why the token ::> was used; 2) If the author is an automatic generator, report a bug for the generator.