I have this same problem David. One of our company files has needed a rebuild before EVERY backup for years. After I do the re-build, it backs up as normal. A second company file has only just started having this problem in the last week. I suspect for this second company, the problem was caused when Accounts crashed suddenly on a computer which doesn't store the data file, so the link was interrupted. After a re-start, everything worked on that computer again but since then I have to do a rebuild before every backup. Did you ever find a solution?
No Sandra not yet. Suspect my file might be just too big. Next financial year I may have to start a new file and archive the previous. My much smaller family file does backup without a rebuild. The main problem is the rebuild prevents auto backup to the cloud.