Events + Fails to add events

One of my multisite subsites (https://www.blackwaterbrake.com, i.e., https://bwb.nettechdata.com) has been utilizing the Events+ plugin to list their upcoming shows. For the past couple months they have been unable to update shows because when they enter start/end dates the plugin defaults to 1970 and won't allow event entry.

This started when I had their site hosted on my Windows IIS server. Last week, I migrated their site to my Linux Apache server, and they just reported to me today that they are still experiencing this issue.

I can't find any documentation for troubleshooting the plugin, so if there is any log file or admin access required to figure this out, please let me know.

The events that are currently posted on the front end of their site are working as expected, but they cannot add anything new. This issue has persisted through plugin updates and WP core updates.