Cannot authenticate against LDAP

I have just installed OpenFire on Linux and have configured LDAP. It allows me to login there and I see all my users in the users list. But, when I launch the Spark client, I cannot login. It eventually says “Invalid user name or password.” Any ideas on where to start looking at this?

This is OpenFire 3.6.4 on Ubuntu Linux (Java 1.6.0_07 and OpenLDAP) and Spark 2.5.8 on Windows Vista 64-bit (Java 1.6.0_11).