OK, I've discovered the answer to my own question, it should be useful information for other people out there. The upgrading process from MySQL 4.0 to 4.1 changed the length of text fields!
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Login Error
You are accessing Koha from a different ip address! Please login again.
This is because the 'sessions' table had its 'ip' field truncated to 5 characters. The other problems had similar causes. I've got Koha working again by dropping all the tables in the Koha database and re-importing the data from my backup. The 'mysqldump' command defaults to creating backups that don't specify character encodings for the tables, so they've all defaulted to UTF-8 on import. I'm very glad I made that backup! -- Kevin O'Rourke ICT Coordinator, National Teachers' Institute, Kaduna, Nigeria 062 316972