This is - basically - true, but I finally solved the problem by using the purge-script from the darktable-toolbox, by deleting the local secondary cache and by using darktable-generate-cache that is coming with DT. That problem was probably related to the other issue I had, caused by using the crawler at startup. Somehow the library got fucked up.
But I think it is safe to mark this problem as solved.
.k