Thanks for the information on the migration from SmarterMail 9x to 13x. You should not see any difference with the calendar events or appointments between the versions. Can you please verify if the calendar xml file has any contents or has the file been 'wiped' or cleared. The default location for the file is as follows:
[ C:\SmarterMail\Domains\testing.com\Users\admin\AE1279A3_calendarData.xml] You will also see the .bak file [AE1279A3_calendarData_bak.xml]. What is the size of the file compared to a possible backup file you may have ? If the backup is larger then you may need to restore the file and see if that corrects the issue.