Actually, the problem has been resolved without my understanding how it happened. My son downloaded and installed a new version of Linux; conceivably that played some part. I had made up my mind to reconcile the account with a make-believe bank statement that would be for a two day period and would include whatever fictitious deposit or withdrawal was necessary to reach the actual start-of-month balance. Then I would reconcile again to the end of the month. Much to my surprise, I discovered when I was preparing to enter the end-of-"month" value, that there was no discrepancy! I was able to reconcile the month against the actual bank statement without any error or difficulty. I've had no problem since.
Thanks for your response; I'll save it in case of a future problem.