Has anyone else seen the above error under Joomla and CiviCRM 5.11.0 when running the Event Report: Attendee List? I've done a bit of troubleshooting and it appears it seems when a custom form value is empty and is then in the form of an empty array rather than an empty string. A workaround is to test for the variable type and, if it is an empty array, replace it with an empty string. There seems no longer to be a Joomla demo site, so can't replicate it.
I plan to raise this as an issue, too.