Post a screenshot of the exact error message.

Here is a screen shot.

More info. I just tried again on PC running an older version of TB (92 I think). It imported fine. After upgrading to 112 and trying again I get this error.

If the import works just fine on v92, then install v92 on the new PC, import events then update to 112. You could also just copy the entire TB profile to your new PC and keep everything as is, unless you only want to keep the calendar events.

The second PC already has 112 installed and is at a remote location. I just need to keep the calendars synced manually.

Is anyone able to help me?

Does the Home calendar of the target PC have existing events before the import attempt?

Both calendars have existing events from previous syncs.

Create a new local calendar then use that as the target for the import and see how that goes.

That might work but it is is not of any use to me as I need to merge the entries between the calendars. Things get added to each of the calendars and need to bring the stuff from 1 to 2 and from 2 to 1 to keep them in sync. I will test it later today just to see. The second PC is at a remote site I am visiting later today.

I don't think that's going to work because of how calendar data is currently stored in Thunderbird. I tried to reproduce the error you're getting, by importing events from the same ics file, twice, into the same target calendar. The first import is successful because the target calendar is empty. The second import fails with that exact error, and the Error Console gives the exact reason why the import fails as follows:

There has been an error reading data for calendar: Home. However, this error is believed to be minor, so the program will attempt to continue. Error code: DUPLICATE_ID. Description: ID already exists for addItem

The problem is the IDs of the events you're importing from the file are already in use by existing events in the target calendar. This ID is a unique identifier for each event, and therefore cannot be shared between events in the same database. In other words, duplicates of the ID are not allowed into the database. This therefore means you cannot simply import the same events on top of each other and overwrite the existing ones, thus eliminating duplicates and keeping only one entry of each event. Not in the current version of Thunderbird that I know of anyway. Thunderbird now uses SQlite as its database for calendar data storage. Perhaps there is an add-on for doing that. You could also file a feature request (enhancement request) to skip duplicate items here bugzilla.mozilla.org if one hasn't already been filed.

You may want to use an external tool to merge the ics files from the two calendars into one ics file, then import that into a new blank calendar. I can't offer any recommendations for such a tool as I've simply not had to do this before. Google Calendar apparently automatically merges duplicates. You could use that and then export the resultant calendar.

Well that is a serious downgrade. Previously it asked if I wanted to skip the duplicates. I does let me select the items I want to import. I will try importing only the new stuff if I can remember which are new. Maybe I will try one at at time and see what happens.

Problem solved. I restored version 91 from my back up image. It works fine now.