Conflict with Event Espresso plugin

For the past few weeks, my event ticketing plugin, Event Espresso, began showing - although intermittently - a “Nothing in Queue” error after users had selected an event they wanted to register for. Here’s my site:

On the advice of Event Espresso support, I deactivated all of my plugins, cleared cache, tried to register again - but still got the error.

Then I switched from Unite to the default Wordpress theme, cleared cache, and everything worked correctly.

I repeated this process again - switched back to Unite and got the error, back to default theme and the error cleared.

Again, this is intermittent but persistent per device. My wife, the business owner, had this error for five days on her browser but not her phone. I’ve had it for two days on my phone but not my browser. Customers are reporting it as well. And we never heard of it until a few weeks ago - everything was fine up until then.

What can I do to fix this issue? Your help is greatly appreciated.