We had this problem on a file that had no unusual characters in its name (we were already aware of the dangers of /), but we noted that the file had a .xlsx extension, and found that saving it as .xls cleared the Read Only restriction, after which, we were able to 'upgrade' the file back to .xlsx. I suspect that the Read Only tag had arisen from the file's closure not being properly recognised, and the system thinking that the file was still open.
A bit more research reveals the following:
- XLS is based on BIFF (Binary Interchange File Format) and as such, the informationis directly stored to a binary format.
- On the other hand, XLSX is based on the Office Open XMLformat, a file format that was derived from XML. The information in an XLSX file is stored in a text file that uses XML to define all its parameters.
- As XLSX is stored in a text file format, Microsoft decided to remove macro support for this file format. Instead they assigned a totally different file extension that allows the use of macros; it is named XLSM. The older XLS file extension does not have this issue and it is able to hold spreadsheets that contain macros or not.
- In general, .xlsx/.xlsm files are somewhat less likely to become corrupt, and can be smaller, but if you make regular backups and have plenty of disk space, there's no reason to spend the time converting.
OTOH, there's usually no reason not to take advantage of the OOXML formats for new files that you create.
- XLSX allows many types of formatting that XLS does not, all of those will be changed or lost.
- Complex XLS files may support 4000 styles, compared to 65,536 in XLSX.
- If you exceed the XLS limit all the others will be lost for sure, but it may be that all formats are lost if you exceed the 4000 limit.