When performing some activity (front-end or back-end) that generates a database action you might get an error that contains the words "returned error: Illegal mix of collations (.....". What is this and how you should go about fixing things?
Ok, such an error indicates that 2 of your database tables have different collations (encoding).
Such errors may have their root cause in bad hosting configurations or other reasons.
These improper database collations show up as errors in SQL JOIN statements between the 2 tables.
The solution is to use phpmyadmin to manually alter the collation of one table to match that of the other.
In your case both tables (and columns of both tables) should have the utf8_general_ci collation.
Google for phpmyadmin and usage instructions for it if needed.