I am receiving the error message ' The Couatl scripting engine has encountered an unrecoverable error and will be restarted. Please download and install the latest add on manager.
You should have seen a prompt suggestion to open the error log as well. That was the most important information we need to explain you what the problem is.
If you missed it, enable Logging in the Troubleshooting section of the GSX Settings and, the next time it happens, exit the sim, check the Couatl.LOG file under %APPDATA%\Virtuali and report back with it's content.
Don't post the log file as a text, but ZIP it and Attaching it to a post, using the "Attach" feature of the forum.