Strange fret buzz problem

Hi everybody. I'm experiencing the following thing. On my 4 string bass, when I press E string on 8th fret, it's upper half (from the nut to the finger) resonates and buzzes when I play C on 2nd string (one octave higher). Otherwise the neck is okay and plays comfortable. When I mute the upper half of the string, it doesn't resonate and there is no buzz. Please could anybody give an advice what can it be and how do I fix it?