I realize this question was asked a while ago but it’s at the top of the forum for some reason. So this is mostly for others who find this question later:
This issue happens when the microflow returns too much data to the browser. Probably in your microflow, some process creates a lot of non-persisted objects or perhaps uncommitted persistable objects. If those objects aren’t necessary to the UI, use a delete action to remove them before the end of the microflow.