This old email from Ulrich Pegelow may be the explanation:
Well, no. The error message appears when darktable is not able to synchronize the two pixelpipes in darkroom mode: the preview one for the navigation window and the “full” one for the center view.
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In some cases synchronization fails, e.g. when the full pixelpipes needs to wait too long and a pre-defined timeout value is exceeded (config variable pixelpipe_synchronization_timeout). If that happens the above warning message is displayed. However, this is of no big concern. Typically with the next processing step all is good again. Any zooming in or out, panning, (de-)activating of modules or any value change is sufficient.