Sort is greyed out for this spreadsheet

I have a spreadsheet, it is not read only, data-sort is greyed out, also sort ascending and descending are greyed out. Adding the buttons again gives the same result.
I applied freeze on Cell A2 to keep the header cells locked while scrolling and applied autofilter. Others have also worked on the document. The document was shared.
To get the sort function back I stopped the sharing, autofilter, freeze, all to no result.
Converting the file to csv and back to ods enabled me to sort again.
Does anybody have an idea what setting may have locked the sorting?
I still have the document where sorting is locked to try.
thanks

Which OS and which LibO version are you using?
Which other formats were involved?

I had the same problem (sort was greyed out) after opening an .xls file (type 97…2003) from the web. As soon as I saved it to .odf format and reopen it, the sort was working again. My GNU/Linux system is Debian 7 wheezy (testing), amd64, LibreOffice 3.5.4.2 350m1(Build:2)

For me, this was because I had enabled Track Changes. Apparently it can’t sort the sheet while tracking changes.