Cause
Possibility 1
A single-line connection definition point contains multi-line connection definitions that do not fit the source or target. For example, the connection definition point was moved or copied to another connection so that the connection points of the new targets do not match the old ones anymore.
Possibility 2
The connection points of the multi-line functions at the targets were changed or the associated multi-line functions were deleted.
Solution
Regarding option 1
Reestablish the associated multi-line functions or their connection points in such a way that they fit the multi-line connection definitions at the connection definition point.
Regarding option 2
Delete the multi-line connection definition.
If required, start a new check run.