Dust and dirt can increase the signal damping of optical cables and lead to false information during transmissions. Itīs recommended, to periodically clean the fibre ends (ferrules). Itīs even recommended, doing this before every usage.
Use a special cleaning pen or cleaning pads. Never use a dry blanket, paper or something like that.
The simple and fast solution: Hama optical fibre pen 5629.
Link to such a cleaning product (PDF):