Thanks Roger and Frank for the suggestions.
I’ve tried to disable Autoptimize as mentioned, tested and no difference. I then deactivated it and hard refresh, no difference. I also disabled CSS and Javascript in the browser and tested, no difference. FullCalendar still wont load.
Tried WP Safe Mode with default theme, still getting same two javascript errors.
jquery.qtip.js:3485 Uncaught TypeError: Cannot read property 'replace' of undefined
at dayOfMonthFormat (jquery.qtip.js:3485)
at Function.<anonymous> (jquery.qtip.js:3485)
at Function.each (jquery.min.js?ver=3.5.1:2)
at o (jquery.qtip.js:3485)
at Object.<anonymous> (jquery.qtip.js:3485)
at e (jquery.qtip.js:3485)
at Object.<anonymous> (jquery.qtip.js:3485)
at e (jquery.qtip.js:3485)
at Object.<anonymous> (jquery.qtip.js:3485)
at e (jquery.qtip.js:3485)
jquery.min.js?ver=3.5.1:2 Uncaught TypeError: $(...).first(...).fullCalendar is not a function
at HTMLDocument.<anonymous> ((index):472)
at e (jquery.min.js?ver=3.5.1:2)
at t (jquery.min.js?ver=3.5.1:2)
Not sure what to try next, anyone else run into this?