Thunderbird 151.0.1 (64 Bit) Windows 10; an error occurred when writing to the calendar

   announceError resource:///modules/CalCalendarManager.sys.mjs:949
   onError resource:///modules/CalCalendarManager.sys.mjs:803
   notify resource:///modules/calendar/utils/calDataUtils.sys.mjs:46
   notify resource:///modules/calendar/utils/calDataUtils.sys.mjs:73
   onError resource:///modules/CalCachedCalendar.sys.mjs:95
   notify resource:///modules/calendar/utils/calDataUtils.sys.mjs:46
   notify resource:///modules/calendar/utils/calDataUtils.sys.mjs:73
   notifyError resource:///modules/calendar/utils/calProviderUtils.sys.mjs:689
   notifyOperationComplete resource:///modules/calendar/utils/calProviderUtils.sys.mjs:662
   notifyListener resource:///modules/CalDavCalendar.sys.mjs:720
   doModifyItem resource:///modules/CalDavCalendar.sys.mjs:794

Hello, I have six calendars: five are hosted by one provider, Runbox.com, via SabreDAV, and one is hosted by a different provider, Apple iCloud Calendar. I am periodically seeing the error shown below on multiple calendars during automatic sync. I have saved a two-day event history from the error console and can provide it on request, along with any other troubleshooting information that may be helpful. I am technical by nature, but my programming experience was primarily with mainframes rather than personal computers. Could someone kindly help me troubleshoot this ongoing issue? Thanks in advance. Bill 10:34:53.958 An error occurred when writing to the calendar House! Please see below for more information. Error code: MODIFICATION_FAILED. Description: If you’re seeing this message after snoozing or dismissing a reminder and this is for a calendar you do not want to add or edit events for, you can mark this calendar as read-only to avoid such experience in future. To do so, get to the calendar properties by right-clicking on this calendar in the list in the calendar or task view. CalCalendarManager.sys.mjs:949:15 announceError resource:///modules/CalCalendarManager.sys.mjs:949 onError resource:///modules/CalCalendarManager.sys.mjs:803 notify resource:///modules/calendar/utils/calDataUtils.sys.mjs:46 notify resource:///modules/calendar/utils/calDataUtils.sys.mjs:73 onError resource:///modules/CalCachedCalendar.sys.mjs:95 notify resource:///modules/calendar/utils/calDataUtils.sys.mjs:46 notify resource:///modules/calendar/utils/calDataUtils.sys.mjs:73 notifyError resource:///modules/calendar/utils/calProviderUtils.sys.mjs:689 notifyOperationComplete resource:///modules/calendar/utils/calProviderUtils.sys.mjs:662 notifyListener resource:///modules/CalDavCalendar.sys.mjs:720 doModifyItem resource:///modules/CalDavCalendar.sys.mjs:794 Here is an additional group of errors from the night before: 23:09:52.245 [calCachedCalendar] replay action failed: <unknown>, uri=https://dav.runbox.com/calendars/908506/bill/, result=2147500037, operation=[xpconnect wrapped calIOperation] 23:09:52.249 calendar: CalDAV: No response status doing webdav sync for calendar House CalDavRequestHandlers.sys.mjs:476:16 23:09:52.249 calendar: CalDAV: No response status doing webdav sync for calendar House CalDavRequestHandlers.sys.mjs:495:16 23:09:52.249 calendar: CalDAV: Error doing webdav sync: undefined CalDavRequestHandlers.sys.mjs:528:16 23:09:52.263 calendar: There has been an error reading data for calendar: House. However, this error is believed to be minor, so the program will attempt to continue. Error code: DAV_REPORT_ERROR. Description: There has been an error reading data for calendar: https://p31-caldav.icloud.com/204954917/calendars/home/. It has been disabled until it is safe to use it. CalCalendarManager.sys.mjs:952:16 23:09:52.263 calendar: There has been an error reading data for calendar: House. However, this error is believed to be minor, so the program will attempt to continue. Error code: READ_FAILED. Description: CalCalendarManager.sys.mjs:952:16 23:09:52.263 [calCachedCalendar] replay action failed: <unknown>, uri=https://p31-caldav.icloud.com/204954917/calendars/home/, result=2147500037, operation=[xpconnect wrapped calIOperation]