Thanks for your reply. Unfortunately this is not the solution. I have tried "track group 2" (although Cisco IOS suggests not to do so) and this is the result:
Router1(config)#module ContentSwitchingModule 2
Router1(config-module-csm)# ft group 1 vlan 5
Router1(config-slb-ft)#no track group MSFC-CSM-link
Router1(config-slb-ft)#track group ?
WORD hsrp group name
Router1(config-slb-ft)#track group 2
% Please consider switching to HSRP group name instead of group number (track group 2)
Nevertheless I have configured it with group number instead of group name on both CSMs. BUT then according to "sh mod csm 2 ft detail" the tracked group 2 stays in state "init" without becoming active or standby on either side.
AND guess what? After config-sync the line "track group 2" vanished on standby CSM exactly like the one with the group name before.