Root issue is plugin loads own version of datepicker which is often incompatible. I would suggest to remove it and use provided library or some other third-party library
Problem: during date poll creation on date selection Datepicker is not displayed, throwing JS-error:
Add additional check to condition:
Send fix to customer to check it solves his issue.
Confluence 6.15.9, MacOS 10.13.6, Firefox 75