Hey Ania, thanks for reaching out! I’m stoked you’ve upgraded to the latest version but am sorry for the trouble here — let’s see if we can clear that error message here together.
First off, do you happen to have any custom templates in your theme folder that may have been used with the older version of the plugin? For example, do you see a folder in your theme directory called tribe-events and does it have files in there? Or how about snippets in your theme’s functions.php file that were used to customize the calendar in the past? Just want to rule those out before we start troubleshooting much deeper.
Also, is the message still there when you activate a default WordPress theme? Try activating the Twenty Fifteen theme that ships with WordPress and see if the message comes up there as well. If it goes away, then that gives us a better clue of where to look.
Let’s start there and see what we find. 🙂
Cheers!
Geoff