I have the same problem as AV-73. Exact same error message. Today (Feb 6th, 2010) was the first time I had experienced the problem. I had the spreadsheets open for several days (probably a week) that I had not attempted to save until today, but couldn't (Error Saving Document XXXXXX. Write Error. XXXXX could not be written). I had done a lot of work with formulas on several different 17 different sheets, so I can't pinpoint anything that I did to the specific spreadsheet. Another thing, I had opened a different spreadsheet earlier in the day and was able to close and save it immediately before I had the problem with the other.
I was able to make a new spreadsheet and copied each sheet into the new spreadsheet and was able to save that file. Everything appears to be OK in the new spreadsheet, but I won't be able to tell until I really dive into working with the spreadsheet again. When I rebooted, Windows restored the original spreadsheet but I lost a lot of the changes, but not all. So I'm not sure when it stopped doing the autosave/backup.
I hope that this is an isolated incident for me, but if not, it is a problem that I need to resolve and am really curious for a solution.
OpenOffice 3.1.1, OOO310m19 (Build:9420), Windows Vista Home Premium Service Pack 2