May be a problem with my setup but after upgrading to the recent 3.1.2 GA I had an odd error during upgrade from 3.1.1. Seems that the Mysql password was lost/mis-set and the mysql_logger_root_password entry was lost from the config.save file. Reported as a null valued key. I added it from a backup and after resetting the MySQL passwords the upgrade completed without error.
Still not getting logger to work, I think the passwords are different as I hadnt any idea what it was set to and it seems encrypted in the config.save.
Reckon I can work through the resetting of the logger in the configs using a search or two in the forums but I was surprised the config.save file could have lost an entry! Possibly down to the backup restore that was done a while back but thought Id mention it just in case it wasnt my fault for once ;-)