Why is my hot water time schedule not active anymore when I overrule the space heating time schedule?

This is the case if the selected hot water schedule is “predefined schedule 3”. Since in this case the hot water schedule is based on the space heating schedule, the schedule will get disabled if the space heating schedule becomes inactive. Because of changing space heating operation mode, the schedule for space heating is disabled. In case this behaviour is not wanted, use predefined schedule 1 or 2.