I have the same problem as the topic referenced.
I use the app on a Galaxy 5 and a Galaxy 5 Note. The Galaxy 5 worked fine straight away. The CalDav server is on Cpanel. No error setting up the account.
The Galaxy Note doesn’t show the calendar in the list of calendars to enable when I look in the settings for S Planner. I cannot add an event to this calendar because it doesn’t show even though it is setup in the app and syncing without error.
I have restarted the device.
Do I need to clear the cache?
Is there a limit to the number of calendars that S Planner supports?
Is there a conflict with either outlook.com / gmail or other accounts (do I need to switch these off)?