I had the same experience as Tony. Removing the plugins before logging in solved the issue, although after enabling them again, things worked as they should. Immediately after renaming the plugins folder back to normal, I did get a strange message on the Plugins page that said:
The plugin ../../../../../../../../../../../../../../../../../../../../tmp/tmpf01R3s/sess_896d8015d80c34ab0f49cc2a88eb2b9a has been deactivated due to an error: Invalid plugin.
I have no idea what this was about, and it went away on its own. Strange.
Here’s how I fixed this issue.
1) Rename plugins folder to plugins.old.
2) Log in and update database if prompted.
3) Rename plugins.old folder to plugins
All my old plugins work without modification, and I can now log in again. Mysterious!
