Links from calendar no longer working

I have an events calendar which has been operating for some years using Events Caledar Pro and Community events plugins. The licences for these were not renewed but the calendar continued to function well until recently. i mistakenly upgraded the Events Calendar plug-in to the latest version, 4.2.6 while the licenced plugins were still at 3.6.1. The links from the calendar to the individual events no longer worked, I realised this was probably because the versions no longer synched.

I downloaded the 3.6.1 version of Events Calendar and manually installed to downgrade. However, the links from the calendar still don’t work, nor does the link to the ‘add event’ form. It just says ‘Not found’. The events are still there and can be found using the search function.

Can anyone suggest what I have to do to restore the calendar? Link to calendar: visithuntly.net