Hey Robin,
Thank you for reaching out to us!
Can you please make sure that both The Events Calendar and Events Calendar PRO are up to date (latest versions are 4.6.1 and 4.4.18)? After that, please access your Dashboard and head over to Settings -> Permalinks for a permalink flush (no need to change anything, just visit the permalinks page: WordPress will automatically flush your permalinks once you visit it).
Be sure to clear your site cache and browser cache after this change.
I hope this helps! Let me know if you have any other questions and I’ll be happy to assist!
Best Regards,
Patricia