by Alain Raap.
Matteo and Ken, good news (and both thanks for all the help), our backup works fine now!
I changed several settings of the NFS client and server and I have made an enormous performance boost with these new settings! I don't see any broken backups anymore (also not in the temp backup folder) so this seems to be the solution for our backup problems.
First problem was the Bluecoat configuration we solved (the progressbar was visible after that changes).
Next I tuned the database settings with the mysqltuner.pl script and with the tips I found here in the forums.
Last I changed the settings of the NFS client/server.
On the NFS client (in /etc/fstab):
[our_server_url]:/share/moodledata /share/moodledata nfs4 _netdev,relatime,async,hard,rsize=262144,wsize=262144,tcp,timeo=600,intr 0 0
On the NFS server (in /etc/exports):
/share/moodledata [our_server_url](rw,async,anonuid=601,anongid=601)