I'm having the same problem (using external mysql):
12:11:00.743 [http-nio-8080-exec-6] WARN o.a.g.e.AuthenticationProviderFacade - The "mysql" authentication provider has encountered an internal error which will halt the authentication process. If this is unexpected or you are the developer of this authentication provider, you may wish to enable debug-level logging. If this is expected and you wish to ignore such failures in the future, please set "skip-if-unavailable: mysql" within your guacamole.properties.
12:11:00.746 [http-nio-8080-exec-6] ERROR o.a.g.rest.RESTExceptionMapper - Unexpected internal error:
### Error querying database. Cause: java.sql.SQLException: Unknown system variable 'transaction_isolation'
### The error may exist in org/apache/guacamole/auth/jdbc/user/UserMapper.xml
### The error may involve org.apache.guacamole.auth.jdbc.user.UserMapper.selectOne
### The error occurred while executing a query
### Cause: java.sql.SQLException: Unknown system variable 'transaction_isolation'
Thank's for your script, just a sugestion if you wont to integrate in next release: add _failed in the name of the failed backup folder destination for a failed backup, that way we know content may not be complete or reliable.
The button 'CHECK FOR UPDATES' in Docker, plugins, ... pages of the web interface, can desapear, and the system check for updates as soon as the page is shown!