Thunderbird Calender

Until today, I was able to export my events and tasks from my desktop to laptop by using the export-import function in the thunderbird toolbar and transferring the file via usb. However, now, when i try to import the file into my laptop thunderbird calender, it fails to update the events and tasks. I get a modification failed message "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."

Any suggestions on this problem and how to fix thanks peter