thank you for the input, we'll check this out and let you know. The contactor is indeed clicking (we can also see the lever moving on the relay).
Initially the problem came in sporadicly 3 or 4 times, and then suddenly it didn't resolve anymore. Your suggestion to look for the brushes will be checked.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.