Losing repeating events after Update from 2.5.15

Hello,

we have the Plugin Version 2.5.15 running on a website and it works as intended, running alot of events – some are defined directly on the website as repeating (weekly) events, some are getting refreshed through the calendar feed option.

After updating, all events that are not in the calendar are being deleted, this seems to especially effect repeating events.

Is there a way to prevent this so we would be able to update the plugin for php 7 compatibility? Its the only one preventing us from doing so currently.