I've been struggling with this issue for a couple weeks with the NPOI 2.0 beta, where the HSSFWorkbook doesn't have the issue, but the XSSFWorkbook does, and I believe I found the source of the problem.
I found that when saving a file in XLSX format, it generated the "Excel found unreadable content" error when opening the workbook. The message was relatively benign, because it was able to open the workbook without data loss; however, it was quite annoying.
In the NPOI wrapper I use, I've always saved a file before doing any work (essentially saving a blank workbook), because I wanted to keep a placeholder. This never created a problem with the HSSFWorkbook. However, it was precisely this action that caused the error.
I changed my wrapper, and my overall strategy to not do this initial save, and that stopped the error.
I just wanted to give this as an FYI
I found that when saving a file in XLSX format, it generated the "Excel found unreadable content" error when opening the workbook. The message was relatively benign, because it was able to open the workbook without data loss; however, it was quite annoying.
In the NPOI wrapper I use, I've always saved a file before doing any work (essentially saving a blank workbook), because I wanted to keep a placeholder. This never created a problem with the HSSFWorkbook. However, it was precisely this action that caused the error.
I changed my wrapper, and my overall strategy to not do this initial save, and that stopped the error.
I just wanted to give this as an FYI