Some months ago I created a site with a couple of pages that included PayPal buttons. They worked fine. Today I tried to replace one of the buttons with a new version, and I got a 404 error rather than a confirmation that the page had been updated.
Other admin functions work fine. In fact, I could update the page in question if I left the PayPal button code out.
I deactivated all plugins without any effect. I tried a couple of plugins that are supposed to allow PHP (even though the PayPal button has no PHP), but without effect.
I'm guessing the WP code changed with an update, but I'm clueless what to do about it.