Errormessage: intersection is undefined for disconnected ranges

I can’t work on my gantt-project, ie. deleting or adding new tasks, delete dependencies etc.
i do not understand the error-protocol and have no clue how to solve the problem. what can i do?

The Problem was solved by unindenting tasks. those had dates in the future of the parents task, which looks like a bug, that has lead to the errormessage. everything works again.