We use your product on multiple sites, using the same theme every time, and have never run into issues like this.
After installing both plugins (event calendar and event calendar pro), I adjusted the settings as normal, then went to create my first event. I then noticed the date selection wasn’t working properly, there was no calendar popup, I couldn’t set recurring events, and the Venues and Organizations wasn’t working either.
So I deactivated all plugins except event calendars, and it was still not working. I then deactivated all plugins, then reactivated all plugins. And now, I can’t even access my events or add a new one! In fact, over half of the Events sidebar menu is missing!! What is going on here! I tried deactivating and reactivating everything again, and switching themes, clearing out the cache from both my browser and web server, and its still broken!