I just upgraded the plugin and the database doesn't seem to have upgraded properly.
Before the upgrade, I didn't have any "description" columns in the table - the upgrade didn't add them but requires them to be there.
There also seems to be a bug in seo-title-tag-3.4.php; line 96 says:
CHANGE description title VARCHAR(255) NOT NULL,
CHANGE description description VARCHAR(255) NOT NULL,
Run these queries on your database (replace wp_ with the appropriate prefix)
ADDdescription` VARCHAR( 255 ) NOT NULL
description VARCHAR( 255 ) NOT NULL
description VARCHAR( 255 ) NOT NULL`