Hey Martin,
Thanks for getting in touch!
Best would 1) making sure that both The Events Calendar and Events Calendar Pro plugins are active and 2) running an update on the calendar plugins.
To run an update go to your WordPress Dashboard > Updates. There you should see a list of plugins that need updating. Select the ones you want to update and click the update button.
The current versions are:
The Events Calendar 4.4.0.1
Events Calendar Pro 4.4
If you prefer you can also update them manually. You can download the freshest versions from your account at theeventscalendar.com under Downloads. https://theeventscalendar.com/my-account/downloads/
Let me know if this helps.
Cheers,
Andras