I'm hoping someone might be able to help me. I have read the other post here created a few years ago regarding this same issue. Unfortunately, I've tried saving the Excel file locally, renaming, and uploading it and the error remains. I've deleted the original Excel file from our SharePoint library and uploaded the saved file, and still when I open the file it forces me to open in Read-Only and continues to give me the same 256 user error, even after removing all permissions and forcing check out in the versioning settings for the library. I also cannot un-share the workbook as the Share Workbook option is greyed out in all copies I've attempted to save.
My organization uses SharePoint 2013, and MS Office Pro Plus 2016. Any assistance would be greatly appreciated!