I have problems opening Catalogs with long catalog names and if it does open the catalog will not backup to the original folder and if I change the backup location it still will not backup.I think this is a Lightroom 2015.8 bug .
How long are these catalog names and how 'deep' is the full pathname? Is something changed latetly in these? Is this on Windows or Mac? Does Lightroom say anything when it don't want to back-up? (like catalog is bigger then 2 Gb)?
You do not say if you are using Macintosh or Windows, but there is a known bug that Lightroom CC2015.8 for Windows cannot backup a catalog with two-byte characters in its name. So it's not the length of the name, but things like è or ü in it.