The main cause of this problem is a sticking or welded relay contact. If problems of this kind arise in your system, then your consumers are causing excessively high switch-on peaks. Due to their input capacitance (up to 10 µF!), energy-saving lamps, ECG devices, fluorescent lamps and similar consumers often produce switch-on peaks of up to 100 A! An unprotected relay contact cannot withstand such current peaks in the long term. This can usually be remedied by using a suitable switch-on current limitation (e.g. EBN 2 or EBN U2) or by connecting the load via a contactor.
If the LED indicating the switching state goes out on the ISK although the connected load remains on, this can be taken as a sure sign of a sticking relay contact.