When logout from PROXY_RDP session, following message was recorded on Windows application event log.
Level: Error
Source: guacd
Event ID: 4096
ERROR [$b4d061b7-6c63-4cba-8c64-907b2d376935] 07/06/2017 14:43:09 - Error handling server messages
This message is harmless, so you can ignore it.