cannot add event calendar front end

Hello,

Please test with the standard Joomla! Template. It sounds to me like your template provider is applying a global setting to some CSS. Or they are loading BS3 and it's conflicting.

Many thanks
Tony

thank you, Tony.
The site is very large, so would be time-consuming to load on a test server. and i cannot afford to make a mistake in loading a default template to test on the live site. so, i have simply disabled the popup function for adding a new event in the front end. that seems to work. however, modifying an existing event still calls the popup function, with resulting dim screen and no popup

In the JEvents Configuration you can set it to Edit In popup, set this to No.

But the original Modal will always use the Joomla! Modal window which is BS2. Your template provider should be able to sort this for you fairly easily if that is the case.

Many thanks
Tony