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It means that the framework thinks a layout phase is already going on when a new phase is about to begin. The cause for this is either that the previous layout phase ended abruptly (e.g. an uncaught exception) without properly marking itself as ended or then there's some part that triggers a new layout phase when another phase is already going on.
The problem could be caused by almost any client-side code that is related to components. If you used any add-ons, could you see if the problem also occurs without them?