Error opening Excel file on SharePoint: "The workbook is currently open by 256 users. A maximum of 256 users can have a shared workbook open at the same time."

The file you are trying to open is, somehow, corrupted - unless it´s being opened by 256 users, obviously. In case of corruption, you can try to save a copy of the file on SharePoint, using the Excel. Ignore the "read-only" warning and "save as" a new file. Delete the old one. Solved!