The relay is not deenergised after the excitation voltage is switched off. What could be the cause? What can be done to remedy that?

This may be caused by a small residual voltage in the circuit, which is sufficient to prevent the relay coil from dropping out. It only needs 0.1*Un to remain energised! With a 230VAC coil, the relay would therefore only drop out at <23VAC!

Long wires/strands often cause induction voltage, which prevents the relay from dropping out. ComatReleco has developed the CEM01 interference suppressor for this purpose. This interference suppressor is connected in parallel to the coil and compensates for the residual current in the wires. This allows the relay to drop out cleanly.