server hangs completely

Hi all, server suse7.2 with kernel 2.4.16-4GB. When starting backup last week, (Arkeia free version, SC30 drive) I experience something new: Server hangs completely, not able to login in any way. In /var/log/messages I can find a line, that is completely filled with ^@^@^@ and so on. In Arkeia logs I cannot find anything, that was not there before also (when evrything ran OK). What do these ^@^@^@ mean?? TIA L1