This will have started happening when the site updated to WP5.5 and you can ‘fix’ it using this plugin:
However, that plugin is intended as a temporary fix to give/the developer some time to fix the code in use on the site.
This is the error being thrown on the site:
Uncaught TypeError: Cannot read property 'msie' of undefined
at HTMLDocument.<anonymous> (custom.js:56)
at i (jquery.js:1)
at Object.fireWith [as resolveWith] (jquery.js:1)
at Function.ready (jquery.js:1)
at HTMLDocument.J (jquery.js:1)
So there’s some updates that need to be done within your theme’s custom.js file to properly fix this.