Well, the test is not good.
After 23 min, I lost the NFS remote share. :x
Tonight, I'm going to do a test by stopping the maximum service...
---Edit---
If I do a "reread export file" (whitout stopping NFS server and restarting), The backup begins again !)
Second loss of the NFS remote share. and do a simple "reread export file"seems not be sufficient I have to also click on the button "start NFS server"