This claim of responsibility was just found at Indymedia:
Last night we sabotaged eight construction machines in the area which is to be prepared for the opencast mining.
At five excavators, two bulldozers, a road roller, and another, expensive-looking thing, all hydraulic hoses and electrical cables that were reached, were cut. The tanks and oil filters were filled with sand, all the windows smashed, and other mechanical components sabotaged. Despite all the security measures taken by RWE, this action was carried out with no problem.