Error "(kablink) Could not open index writer" after upgrade to Filr 4.0

After an otherwise successful upgrade to Filr 4.0 from a Filr deployment which started as an early version (e.g. 1.2), a user can login as vaadmin and as root on https://<IP>:9443. But when attempting to login as admin on https://<IP> or https://<IP>:8443, an error appears:

(kablink) Could not open index writer

1. Remove the old index directory (assuming the old directory is already backed up as backing up the vastorage disk is a necessary step for the upgrade to Filr 4.0.)

2. Restart the Filr service.

The problem is caused by a Lucene version mismatch, as reflected in the /var/opt/novell/tomcat-filr/logs/appserver.log.