Datepicker returning uncaught typeError: undefined 'currentDay'

The above jQuery datepicker error is generally caused by having a duplicate controls with the same ID, no matter whether you use id or another selector like css class or field name to instantiate it.

I solved this with create new ID for HTML DOM object. There is duplicated Id. Try to add new ID for HTML DOM object.

I've found the solution. After a long time of debugging I figured out that there was a <div> that had the exact same ID, lying higher than the input field. Therefore the script took the first instance that contained the ID and picked the DIV instead of the input field.

I removed/renamed the DIV and it worked fine again.