Thanks, Sara. No, I didn't use any unusual characters. I just went back to "Modify Account" and the password change was accepted. Don't ask me why it didn't work last night.
Somebody broke into my eBay account and listed two items for sale on eBay France under my userid. I cancelled the auctions (one of which had 15 bids on it already) and got eBay to credit my account for related charges, but it was a major lesson to me to change my password every now and then. EBay says they have no way of tracing the miscreant who used my id.