You've not stated which OS you're running or the TB versions (both old and new) so it's difficult to help - if you're running Windows, are there any files in the calendar-data folder located at C:\Users\[Users]\AppData\Roaming\Thunderbird\Profiles\XXXX.default-release on each of the old and new PC?

Old and new for TB and Windows 11 are the latest versions (auto update).

Thanks for your reply. For some reason I can't paste a screen shot of the Profile folder here, but the new folder has four files:

deleted.sqlite 96 KB local.sqlite 3072 KB local.sqlite-shm 32 KB local.sqlite-wal 3651 KB

and the old folder has exactly the same.

Same problem here. Imported a Thunderbird 115.18 profile to Thunderbird 128.11 on Linux Mint 21.3. Ten GB mail imported successfully from original profile, but no calendar entries show up. The local.sqlite file in the profile's calendar-data directory is the same date and size as the original, but TB 128 calendar is empty of entries. Anyone have a solution?