user unable to access "crontab -e"

Following an update from bookworm to trixie on one system, my regular user 
account is no longer able to edit its crontab file on that system.

When I execute "crontab -e", I receive the message: "No modification made" and 
an editor opens on a blank file.

The user's crontab file does exist:

sudo ls -al /var/spool/cron/crontabs shows:
   -rw------- 1 n7dr crontab 2981 Sep  2  2023 n7dr

and, indeed, that file seems to be being processed correctly by cron. But I 
just can't read and edit it with "crontab -e" :-(

I have no idea why this system is different: the problem doesn't occur on two 
other systems that have been upgraded.

Suggestions welcome.

   Doc