Reply To: Calendar not showing – "-1" displayed instead

HTTPS and easyreservations have nothing to do with each other. easyReservations just answers requests and doesn’t care which way they came. It’s like easyreservations is a mailbox and you say it’s incompatible to a mailmen just because the mailmen doesn’t deliver your mail.

Something in your wordpress and/or server configuration is wrong. For https the most common is to not enter https:// in the siteurl options in wordpress general settings.
Regards