I just ran into this too. In my case the &errors addition to the URL didn’t help, so I had to start hacking around in /pepper hoping to find a clue.
It turned out the problem for me was a residual unzipped “default_v209” folder that I had left at “/peppper/default_v209” while upgrading in the past. (As a peer to e.g. “shauninman” namespaced folder.
Will take care to keep that folder more tidy in the future!
(Update: turns out my errors not showing was specifically because I’ve configured my site to log PHP errors instead of showing them. Duh!)