I'm not quite sure where the problem lies with you, but I had a couple things to consider when dealing with character sets when I migrated to Plesk from another control panel when dealing with Korean text.
First, how does it appear in the database if you check it out with phpMyAdmin? Is it correct, or at least how it used to show up? If not, it may be a collation error. Somehow for phpbb Forums I got Korean text working, but it was only working with the Swedish collation since that is what it was initially set up for. When I migrated, I wanted to use utf-8 for everything to make things simpler, but the forums had to stay Swedish or else all the old posts were lost.
Another issue can arise if you are accessing the database using php. For the majority of my content, it uses utf8_general_ci collation (I believe that's what it's called). However, when accessing using php, it used to grab it using a western charset for some reason. I had to add a line of PHP to set all mysql queries to utf-8 charset.
I don't know if any of this is helpful for you as you are simply upgrading in Plesk, but something may be different involving one of these with the updates. From my experience, how you store data in the database (using PHP), how you retrieve it (using PHP), and the collation of the database all need to match, or you may run into errors (with minor exceptions like my Swedish issue - that had to stay the same, yet the text shows up properly in phpbb forums - must be in the phpbb code how it retrieves the data as I'm not fully aware of all the code involved for those forums)
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