You have 2 items with the same name:
Group Tempus "Temperatur" <temp> (Home) // This Group collects all Temperature-related items, Indoor and Outdoor
Group TempIndoor "Haustemperatur" <temperature> (Home) //This Group just groups all indoor temperature sensors.
Group:Number:AVG TempIndoorAVG "Durchschnittstemp Haus" <temperature> //I'd like to get an average of all indoor temperature items
Make the indoor temperature items member of the group TempIndoorAVG
OR
Remove the AVG group and make the TempIndoor group an AVG group:
Group Tempus "Temperatur" <temp> (Home) // This Group collects all Temperature-related items, Indoor and Outdoor
Group:Number:AVG TempIndoor "Haustemperatur" <temperature> (Home) //This Group just groups all indoor temperature sensors.