Just a thought, but the starboard side on the first photo looks relatively clean, and the report from elgan is of an engine fault.
Maybe the mess down the port side is the result of an oil leak, plus a severe overnight frost? Looking at the aft end, the stain gives the impression of being feathered by the slipstream, and the stains all emanate from behind the exhaust.
My twopen'th, giving BI a bit of slack
