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Loved it, used it. But since yesterday I get a message that the certificate has expired october 2nd

Ta opinia jest o wcześniejszej wersji dodatku (1.0.9.8). 

I contacted Telenet and will correct the module if they do not renew the certificate.
In the mean time, you can go to https://telemeter4tools.services.telenet.be/TelemeterService and add a permanent exception for the certificate (if you're using Firefox 3).