What were the exact steps when you exported the calendar on the Windows computer? How did you transfer the file to the Linux computer?

Under Calendars select Home, three dots on right, Export Calendar, save to a Home.ics file. The PCs are on a network. The Linux one can see W10 PC and so can select the file using Tools, import... I have tried saving the file into Linux in a separate location from the network link but get the same result.

That sounds reasonable.

When you open the .ics file copied to the Linux computer with a text editor, do you see anything unusual? You should be able to recognize existing calendar entries.

I hadn't thought of doing that. Looks like each day has a separate entry, with nothing obviously wrong. Typically :

BEGIN:VEVENT CREATED:20250910T163309Z LAST-MODIFIED:20250926T162746Z DTSTAMP:20250926T162746Z UID:b3317722-8766-42f4-95f9-6c65b099bd59 SUMMARY:RC at new unit CATEGORIES:Repair Cafe DTSTART;TZID=Europe/London:20251003T100000 DTEND;TZID=Europe/London:20251003T130000 TRANSP:OPAQUE LOCATION:Cascades X-MOZ-GENERATION:3 SEQUENCE:1 END:VEVENT

Maybe it can be imported differently. Thanks Alan

I have created a new calendar in the W10 PC with a single event in it. This imports into Linux OK. So maybe the larger file is too big to import? However when I added 4 test items to the file for export I got this message when importing:

"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."

Setting the new calendar to read-only allowed the import to succeed. With lots of old events in the original calendar there could well be much to trip up on. It's going to be easier to start again from scratch.

This sounds somewhat odd. It's not something I could explain. In any case, is your problem fixed now?

Hi, thanks for you interest. I tried the read-only approach with the original large file, but that didn't work. On reflection, there's not much point in copying all the old dates. Alan

This is hopefully my last bash at this. I deleted all past events in a text editor, leaving a lot of initial entries which seem to relate to daylight-saving changes in past decades. Repeating events such as birthdays appear near the end of the file. It now imports correctly!