Java error in the selection of the pool
Under "Source Options" on the last page of the Restore dialog you will see 2 lines.
"Restore from original pool" with an empty selection list (this returns the Java error)
Restore from alternate pool with a choice of pools
Cause: different backup server names "Default Address" and "Hostname"
Workaround 1: use the Alternate Mediapool in the Restore Options.
There, select the original media pool that you see in Device in the Restore Point Selection.
Workaround 2: set the "host name" to the value in node management for the backup server,
which is registered at the "Default address".
So GUI-> NodesManagement -> "context menu on backup server" -> EditNode-> ConfigureAddress-> ManageAddresses.
Workaround 2 is permanent.
It causes the original Mediapool to be put back in its designated place and the restore to continue as before.