Good afternoon Greg and welcome to the Events Calendar forum!
Thank you for your interest in our products.
We’ll be glad to answer your questions.
To your point, we are alas limited to the level of support we can provide in our Pre-sales forum unfortunately.
For that reason, I would highly recommend asking your client for their the Events Calendar customer info.
With that in mind, it sounds like the theme you are using also has some custom Javascript being triggered (the black page) when clicking on the next month link. Try temporarily reverting back to a default WordPress theme such as twenty-sixteen (with a “maintenance” page plugin in place) to see if that changes anything.
You could also try different settings for Events Template under Events -> Settings -> Display.
I wish I had a better answer for you, but for now it’s the best one I have alas.
Hopefully we can take this further in the premium forums with you.
Best regards,
Geoff B.