I have installed Thunderbird on a new computer, created an empty calendar and want to import the calendar backup of the old computer (ics file). After approx. 50 percent of the calendar entries (approx. 700 in total) have been imported, the import aborts with the error message “Modification_failed” The following note appears in the error message An error occurred while writing to the calendar Wolfgang! Further information can be found below. If this message is displayed after using “Reset” or “Close” for a reminder, but you do not want to add or edit any appointments for this calendar, you can mark this calendar as read-only so that this message is no longer displayed. To do this, right-click on the calendar in the calendar list of the appointment or task view and open the properties. I had switched off the reminder function in the empty calendar