Linksys WRT54G firmware 4.21.1 is not backward compatible

I have a Linksys WRT54G wireless router, hardware version 1.1, used
daily since early 2004. I recently upgraded from firmware 4.20.7 to
firmware 4.21.1. When I use the Access Restrictions tab to enable/
disable one of the PCs in my home, an operation done successfully for
many months, and then Save Settings, I see "The values you entered are
invalid. Please try again." There is no further explanation of what
the problem is, and I know the settings I am trying to save worked
before. I went back to firmware 4.20.7 and the exact same settings
are accepted with no problem. I plan to stay away from version
4.21.1. A web search shows other users unhappy with this firmware
also, such as the following currently-unanswered post on the Linksys
website: http://forums.linksys.com/linksys/bo...hread.id=45831.