Hello,
Thanks for reporting this issue. We’ve seen it happening other times, and it’s caused by a third party plugin that still relies on the jQuery UI datepicker calendars.
Basically, a third party plugin is re-declaring the disabled dates, and this turns out into an invalid setup for the datepicker calendars.
This is something that could be debugged by looking at the HTML source code of your web page, but our suggestion is to disable third party plugins that you don’t need, because they should not be loading JS assets on pages where those plugins are not being used. Our plugin Vik Rent Items never loads CSS or JS assets on pages where there are no contents generated by our plugin, and this is what any other developer should do to avoid conflicts with other plugins in the first place, as well as to avoid loading unnecessary JS assets that just make the page heavier.
We invite you to reach out to our technical support team should you have any problems, so that we can check the HTML source code of that specific web page on your website to understand what third party plugin is interfering with the jQuery UI datepicker calendars.
We hope this helps!
The VikWP Team