tickets that are related to each other block a declaration of parent task for them (error: invalid task)
Ticket A has a relation to ticket B.
Ticket C should be a parent task for A and B.
So I create C.
Then I try to add parent task C to A and B.
Redmine tells me that the parent task is invalid.
Then I remove the relation between A and B.
Redmine allows me to add C as parent task to A and B now.
Now I can re-add the relation between A and B again.
Result: A and B have a relation and they have C as parent task.
But: Why cannot I add the parent task C to A and B while they have a releation?
IMHO this should be possible without the detour removing the relations and adding them again.