Mail preview pane disappears

I periodically back up my Mail 2.1.3 folder from my iMac G5 (Tiger) to MacBook (Snow Leopard) and have had no problems until today.

Problem: All my Inbox messages are listed in the backup, but the Preview pane for those messages received since June 2015 is blank. Preview pane is present for all messages received prior to June 2015.

Rebuilding the Inbox causes the number of messages to jump from dozens to thousands and the problem remains, likewise reverting to yesterday's Time Machine backup is no solution.

Can anybody suggest a reason for this bizarre behaviour, which is totally new in my experience?