I found this forum post, where someone appears to have the same error. Multiple answers are suggested though none are confirmed. Maybe one of those will help you? If you upgraded the excel importer recently, make sure you follow the steps mentioned in the documentation when doing so.
For me the solution was to save the Excel file as xls, not as xlsx!!
I just had this same error.
My issue was that I had microflow calling itself rather than a sub-flow, so causing an infinite loop! Annoying… :D