The article says that due to a construction error (or design error, the term can mean both things), water has been accumulating underneath the rails of the coaster, causing rot. Presumably, it's the running strips of metal or possibly the glue laminate that causes the problem: water can seep into wood through capillary forces, but the structure needs to be able to dry out again to prevent rot. For one reason or another, the coaster rails were not able to dry properly, and rot set in. The cost of replacing the rails is quoted as 30 million SEK, or about £3 million.