Try this and see if it's helping:
Open your ****.gpx file (pois with Cyrillic characters) with windows Notepad.
Go to FILE-->SAVE AS and choose Encoding UTF-8 and then press SAVE.
Open again the ****.gpx file in Notepad and in the first line find and replace: encoding="ISO-......." with encoding="UTF-8"
Go to FILE-->SAVE
Now open your gpx file using Mapsource or Basecamp and see if the the Cyrillic characters appear correctly.
If its OK then use Poiloader to create your .gpi file.

This is a common problem with Greek characters and old servers where the files were encoding with ISO-8859-7
It also depends on your language settings of your windows PC and which codepage you are using.
It is a very confusing problem, especially when you download a .gpx file with ISO-.... encoding, add or change something with a PC with UTF encoding and save it.