This happened on two separate installations. Large-ish image uploads don't work anymore, and I can't backup my db using a db backup plugin.
Both installations are running multisite. I deactivated and re-activated all plugins on both. I checked both to make sure the memory limit was set in wp-config (it is). There are php.ini files in /wp-admin. On one, I deleted wp-admin and wp-includes entirely and uploaded a fresh install.
At first I didn't think it was necessarily related to the upgrade, until the exact same thing happened on another installation. I haven't seen this issue reported here yet. Thank you for any help you may have