I did that and no more question marks, but I get something like this appearing in the CPG- години 17 класации.
I now noticed that when I am not logged in I am not able to view the full size of the images even though this is allowed through the configuration. I get a database query error.
Probably is best to give you access to a test account where I am working on this, or do you prefer a copy of the DB, there is more then just one table broken, but mainly the comments are the problem, as the rest of the data is not that much and can be manually modified.
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If by the 'front end' you mean CPG - one additional change to try...
You changed the tables to UTF-8... but sometimes we have seen that MySQL doesn't correctly open the connection (varies by host, MySQL defaults, and maybe other reasons...)
In include/config.inc.php add the following line (location doesn't matter)
$CONFIG['dbcharset'] ='utf8';
This will force CPG to explicitly tell MySQL the data is encoded as should be treated as UTF8 when the connection is opened...
If that doesn't work - I would like a copy (backup) of your categories table from BEFORE any changes were made to do some testing in my sandbox... (unless Αndré has another approach since he was already working this...)