Hi Karsten Lamest,
In any usertask there are two microflow you can set.
1st one is for target user , in this microflow you have to return list of users whom your workflow task will be routed to, in this microflow you can perform other activity as well but it should return user list
2nd one is event microflow of usertask , it could be useful when you want to perform some action after your workflow task has been routed to users but it is not mandatory.
Now error you are getting seems in 1st microflow you are committing something but not returning any user list which is required as it will route task to those users.
Hope it helps!