Illegal mix of collations
This is for all errors that look something like
Illegal mix of collations xxx and yyy for operation ‘=’(Errno=1267)
This is an error generated by MySQL when several fields in the database don’t match each other.
This can happen when you move your blog from one MySQL server to another (which might use a different default collation) and then upgrade (sometimes you don’t even need to upgrade).
To fix this, you should connect to your database using phpMyAdmin and carefully check the collation of all tables and all text fields in your tables. Changes those that don’t match what you want.
If in doubt, change all your DB encoding to UTF-8, by following the instructions in this page: Upgrading your database to UTF-8.