Calc doesn't keep references to other files.

I’m using Libreoffice 4.1.4.2 on Linux, and use several files referenced for calculation. They work fine while the file that collects all references is open.But after I save and reopen the file, references are lost (I get a warning when it opens, detailing every file reference lost). Suggestions?

Please can you show a formula with link(s) that fails on opening?