After some detective work we found that some pipe had been pinned by one user which caused the error. Even if recent work was not near the pinned pipe, eventually the connectivity of the pipe would lead back to the pinned element and cause the error. The solution was to go to a 3D view, select all the pipe and pipe fitting, then type UP for unpin elements. After that the save went through.
Just guessing here but pinning any element with connectors seems like a bad idea when work-sharing is enabled.