Hi Martin,
Thank you for reaching out to us! I’m very sorry about this issue you are experiencing.
Can you please follow this tutorial to determine if a plugin or theme conflict is happening and find out what is causing it?
Also, can you please enable WP_DEBUG and WP_DEBUG_LOG and share with us any messages you see while accessing your calendar? All WP_DEBUG messages will be stored in a debug.log file in /wp-content/debug.log.
Let me know your test results and I’ll be happy to further investigate if necessary!
Regards,
Patricia