Please make sure that your QOwnNotes is at least at version 16.07.9, you are using Qt version 5.5 or newer and can connect to your ownCloud server in the ownCloud tab in the settings.
If that works and you still don't get any calendars when you press reload calendar list in the Todo tab in the settings please follow these steps:
- close the settings dialog and enable the log dialog by clicking Show log dialog in the Window menu
- make sure the Info checkbox is checked
- press Clear log
- leave the log dialog open and open the settings dialog again
- press reload calendar list in the Todo tab
- close the settings dialog and copy all output from the log dialog to the clipboard
- create a new issue at QOwnNotes issues and paste your log output there, along with the output from the Debug tab in the settings
I will then try to find out what is happening, thank you for your help!