> autonomy.deanst.rroom.net /mnt_c/Autonomy comp-root-tar-win

> define dumptype comp-root-tar-win {
>    root-tar
>    comment "Root partitions with compression"
>    compress client fast

You are using a client specific for Windows, and you are doing the
compression on the client, that is on windows, using windows software.

On the other hand:

> amrestore /dev/nst0 autonomy.deanst.rroom.net '/mnt_c/Autonomy$'

You are restoring on your server, doing the uncompress on your server,
on Unix. Apparently the compress software on Windows is incompatible
with the uncompress software on Unix...

You'd need to extract the data from the tape and then find the proper
uncompress client to use on Unix or push the file to your Windows
client and uncompress there.

best regards,

Olivier