BobIsRight

Setting a new memory limit in wp-config fixed it. The clue was looking at the debug output.

it turns out I needed to increase the memory limit.

There is clearly some interaction with the plugin’s. I agree that 3 having the same problem is unlikely, but I have to start poking around with the data I have and that says the blog fails when the plug-in is active. If you have other thoughts on where to look, I will gladly consider them.

That said, activating the contact form by itself, as the only plugin, does not cause the problem, now that I look at it.