You may in fact be able to solve this problem yourself. Can you please try the following:
1. Open the following file in a text editor: C:\Program Files (x86)\Complete FTP\Server\Server\CompleteFTPService.exe.config
2. Around line 44 you should see a line starting with: <appender name="Audit"
3. Underneath this line add a new line with the following text: <encoding value="utf-8" />
4. Save the file
5. Delete (or rename) your current audit log file at C:\ProgramData\Enterprise Distributed Technologies\Complete FTP\Logs\Audit.log
6. Restart the service
7. Try uploading another file containing Cyrillic characters