Thank you for your support!
It now works without losing some switching commands.
I have adapted the Gateway software according to your suggestions.
And in the fhem, I defined the devices as follows:
define MYSENSOR_10 MYSENSORS_DEVICE 10
setuuid MYSENSOR_10 5c61b043-f33f-3ee4-4a0b-da4876cebf4faa94
attr MYSENSOR_10 IODev meinMSG
attr MYSENSOR_10 mapReading_id4 4 id
attr MYSENSOR_10 mapReading_id5 5 id
attr MYSENSOR_10 mapReading_id6 6 id
attr MYSENSOR_10 mapReading_id7 7 id
attr MYSENSOR_10 mapReading_power 0 power
attr MYSENSOR_10 mapReading_power1 1 power
attr MYSENSOR_10 mapReading_power2 2 power
attr MYSENSOR_10 mapReading_power3 3 power
attr MYSENSOR_10 mapReading_status 0 status
attr MYSENSOR_10 mapReading_status1 1 status
attr MYSENSOR_10 mapReading_status2 2 status
attr MYSENSOR_10 mapReading_status3 3 status
attr MYSENSOR_10 mapReading_temperature4 4 temperature
attr MYSENSOR_10 mapReading_temperature5 5 temperature
attr MYSENSOR_10 mapReading_temperature6 6 temperature
attr MYSENSOR_10 mapReading_temperature7 7 temperature
attr MYSENSOR_10 mode node
attr MYSENSOR_10 requestAck 1
attr MYSENSOR_10 room Heizung_OG
attr MYSENSOR_10 setReading_power 1
attr MYSENSOR_10 setReading_power1 1
attr MYSENSOR_10 setReading_power2 1
attr MYSENSOR_10 setReading_power3 1
attr MYSENSOR_10 setReading_status off,on
attr MYSENSOR_10 setReading_status1 off,on
attr MYSENSOR_10 setReading_status2 off,on
attr MYSENSOR_10 setReading_status3 off,on
To control the relays individually, I defined a "readingsProxy" for each relay:
define Relais1 readingsProxy MYSENSOR_10:status
attr Relais1 devStateIcon on:sani_heating_level_100@red:off off:sani_heating_level_0@#18B8ED:on
attr Relais1 group Heizungskreise,
attr Relais1 room Heizung_OG
attr Relais1 setFn {($CMD eq "on")?"status on":"status off"}
attr Relais1 setList on off
Thank you!