The plugin might be not compatible to your LimeSurvey version.
framagit.org/SondagePro-LimeSurvey-plugin/renderMessage
The information about compatibility is not often accurate.
If you e.g read 2.50+ or higher, it doesn't mean LS 3.0 is working. LS 3.0 is higher, but not compatible to LS 2.50+