Error, then blank pages after upgrade

I tried to apply the most recent upgrade (from 9.6.0 to 9.6.1p1) following the upgrade guide on the wiki.

At first, I got the following error:
The request could not be completed due to too many unsuccessful forward attempts.

Now I get blank pages, even for the manager.

Any ideas?

Save your original config.inc.php file and re-upload your 096p1 files to your server. Put your config back in place and re-run again in upgrade mode.