Not sure if this is the same issue, but last time this happened I've discovered that the default calendar of the user disappeared from the user's folder.json:
Here is what I sent to ST support after finding out about it:
As our customer was stressing about this, I took the time to understand how folders.json and the different folder-xxxxxx.json works and finally discovered the problem.
I did a difference check of the users folders.json file from the backup before 7142 update and after the update.
in folders.json from backup before the update contains this entry:
"display_name": "My Calendar",
However the user folders.json from after the update, this entry is gone!
I added it back to the user current folders.json and now the calendar is working again.
Now we hope that there isn't other users with the same problem. Do you have by any chance a tool to check the user against this kind of integrity problems ?
So maybe the first step to see if this is the same issue, you could check the user folder.json file to see if there is an entry named ""display_name": "My Calendar","
If this is missing, it's probably the same issue. I fixed it by adding back the entry from a backup of the file and calendar started working again.
edit: if you have to restore the entry, be sure to take it from a backup of the user folder.json file, I guess the ID's are important so you can not use some random values here.
edit2: For ST team, if you want to check the ticket I had opened for this, it's the one containing 2550C0C2 .