Circular relation

Martin,

I haven't seen that "No calculation can be performed..." error since at least MSP 2003.  More recent editions of MSP make it very difficult to introduce circular logic through the user interface.

Most circular logic mistakes in MSP result from assigning logic to summary tasks - or more precisely, mixing parent-child task inheritence logic with explicit sequential logic.  Your paranthetical remark implies this may be the case for you.  The easiest and best solution overall is to remove all predecessors and successors from summary tasks.  (E.g. Temporarily insert columns for "Predecessors", "Successors", and "Summary" into a bar chart view; Apply a pull-down filter to show only tasks where "Summary" is "Yes;" then clear predecessors and successors lists for all the summary tasks.)  

In the absence of summary logic, you could manually trace down the sequencing mistake using the task detail form (predecessors and successors shown) or the network diagram view.  I use a logic-tracing add-in of my own creation that does the job handily using a truncated logic trace. I wrote about it here: http://wp.me/p6CCB4-82.

Tom,

Meanwhile did more or less what you suggest in your reply. I found that I had lets say Summary task A as a predecessor of summary task B and meanwhile I had also sub-tasks of A being the predecessor of sub-tasks of B. Once I had cleaned that up the message did not re-appear. So leason learned there.....!!!