Error codes using 1Cal on Android phone

Have recently 'lost' 2 of my 3 calendars from Android version of 1Cal. I connect with iCloud Calendar which still shows all 3 calendars.

Error code: Response status code does not indicate success: 500 (Internal Server Error).:<?xmi version ="1.0" encoding ="UTF-8" standalone = "yes"?><multistatus xmlns ="DAV":">

I have tried uninstalling and re-installing but problem persists.