Current version doesn’t work with WPMU, only common WP
Existing scheduler is oriented on single user usage, while MU is multi-user installation, which requires different handling.
Support for WPMU and multi-user scenario in Joomla is our next goal for plugins.
WPMU uses different approach to table naming, which prevents usage of current version of plugin.
I think, that update, which will re-enable basic support of WPMU will be available on the next week.
Get a guaranteed answer from DHTMLX technical support team
under the most suitable support plan