“Trying to perform an illegal action”

I’ve been using the free versions of Events Manager and WP FullCalendar to reserve conference rooms, but lately this setup is having issues. Sometimes it will work fine, other times users get “Trying to perform an illegal action” when they submit a booking. It seems like logging into WordPress and then having the user refresh the page makes it work temporarily. I don’t even have to change settings, just touching the WordPress dashboard somehow fixes it. This is happening on multiple sites (all on Bluehost). All of them have the latest updates to WordPress and the plugins, and these are the only plugins being used other than Bluehost’s management plugin.

I’ve looked through just about every other booking plugin, and this is still the only one that does everything I want. I’m wondering if the pro version is more reliable? I’m willing to pay for this if it stays more stable than the free one.