ARCServe 2000 - E3702 Unable to connect to group (GROUP=<none>, EC=NON EXISENT GROUP)

I am a completlly newbie in backup ops. I try to do a differential backup to an HP DAT. All backups ops are going well. When I am trying to restore a previous file a get a pop-up window asking me to insert a specific tape and in the log file I get this error:

E3702 Unable to connect to group (GROUP=<none>, EC=NON EXISENT GROUP)

The whole thing goes like this:
I made a folder with two files. file1.txt and file2.txt
I backup the folder (FULL Backup)
I changed the file2.txt
I backup the folder (Differential)
I changed the file2.txt again,
I backup the folder again (Differential)
Now I want to restore to the 2nd backup and restore only the changed file2.txt. When I select the desired file from the Options -> History, I choose restore and after a few secs I get this error.

Any ideas?