OK Nico,
I spoke with one of the devs, and the thinking is that since you are in the high ASCII char set already, that the updated is converting your UTF-8 encoded characters over again. There is a workaround, but I must stress that you try this on a test environment first.
You will need to start at the beginning with 1.6.6 again. Once you have that up and running, before you run the update to 2.4, you will need to edit a file.
In your copy of 2.4, go to /system/expressionengine/installer/updates/ud_200.php
Find:
foreach ($tables as $table)
{
$progress = "Converting Database Table {$table}: %s";
$count = $this->EE->db->count_all($table);
$offset = 0;
It should be around line 415. No comment out the code beginning with:
/*
if ($count > 0) //around line 421
... all the way through ...
$offset = $offset + $batch; // around line 465
}
*/
Then upload this to your test server and perform the upgrade per the instructions.
What this will do is convert your tables to UTF-8 collation but not the data in those tables. Which is OK, because using Polish and Russian, the data is already UTF-8 and we don’t want to double convert.
I look forward to your reply!
Cheers,