Excel 2007/2010 can't open .xlsx files with sheet names longer than 31 characters. It looks like a regression introduced in r993246, see Bug 49887 See discussion on poi-user: http://markmail.org/message/lxscumtoiq2hrjdz Yegor
I can take a look at this if you haven't already made the changes. Jon
Fixed in r1138819 I fixed workbook.createSheetName(sheetName) to silently truncate the input argument if it is longer than 31 character. The idea to throw IllegalArgumentException is not very good. My concern is existing usages of POI in production. With version of POI prior to 3.8 existing code swallowed long sheet names without any exceptions. If we decided to throw IllegalArgumentException then these system would start to fail which is not good. Truncating the input argumnet to 31 character seems a reasonable workaround. Yegor