Hey Martin,
The troublesome line can be seen in full at this link → https://github.com/moderntribe/the-events-calendar/blob/release/117/lib/tribe-templates.class.php#L688
So it seems that the $post variable isn’t an object, which means that the $post global isn’t being set properly or something – there are several things that could cause this, and if you are truly interested, you should run through the full set of troubleshooting steps we have outlined in this article and check on this issue after each step, and see if anything improves along the way → https://theeventscalendar.com/knowledgebase/testing-for-conflicts/
The [relatively] good news, however, is that this is just a “notice”, not an outright error, so it really shouldn’t be causing any actual problems with functionality on your site – is this true? Or do you find actual problems when you try to view or use your events, as well?
— George