You can get LDAP-specific debug information by selecting Yes for Enable Debug in the setup tool's Step 3: Connection Settings page.
You can also get broader debug information by turning on verbose debugging inside the application. Jive SBS provides a fair number of debug messages. To enable this, turn on the debug log via the Log Viewer in the admin console. (In the console, go to System > Management > Log Viewer, click Enabled, then click Update. You'll need to restart the application server for this to take effect.) Due to the large amount of debug information this can generate (and the performance impact that has), you should run this only while developing or testing.
Once you've enabled debug messages, watch the jive.debug.log file. It should describe the steps it's going through to load users and authenticate them, as well as any errors it might run into. (You can view, download or email the debug log from within the admin console at the Log Viewer page described above.)