To get TrueCrypt to work over a network, you need the mount option "allow_other" (and also setting "user_allow_other" in /etc/fuse.conf). I had the same problem when trying to access a TrueCrypt volume on a USB stick attached to a thin client (LTSP = Linux Terminal Server Project, often associated with Edubuntu). In my case the network file system is "ltspfs", for others it is sshfs or nfs. Check what it is for you.
This is how I restarted ltspfs with the "allow_other" option (which mtab showed was missing):
(The IP address was assigned to the thin client by the DHCP server.) Now TrueCrypt can open the .tc file, and "mount" shows:
sudo ltspfs 192.168.1.24:/var/run/drives/usbdisk-sdb1 /media/myusername/usbdisk-sdb1 -oallow_other
I have not figured out how to have "allow_other" automatically be set when mounting the USB stick over the network.
ltspfs on /media/myusername/usbdisk-sdb1 type fuse.ltspfs (rw,nosuid,nodev,allow_other)