Thanks for the feedback folks. For those of you who wanted more information, I left it out because there really isn't anything. I have an account on my server that I create standalone installations of plone using ./install.sh standalone --target="path" for testing. I do everything locally and then for final testing before moving the installation to its own account I set up proxying through Apache. Suddenly, the new test setups stopped responding when put behind Apache. I could still access them fine locally, and older installations set up this way still worked.
I've turned on proxy tracing and looked through the Apache logs. I get no errors when running the instances that are giving problems in the foreground. I am still mystified.
One Clue
I created a new user and installed another instance in that user. The installation in the new user account works fine behind the proxy. I assume this means that something about the way Plone gets setup within an account is messed up in the one I was using for testing. Is there somewhere that an older Plone installation might have left something that would impact newer ones? I thought everything was self-contained within the specified target folder. I would like to get my original testing account working again. But my problem is solved by the creation of this new account.
Thanks.