Hi Micha,
Are you trying to backup or to restore the backup?
If you are trying to backup and get that error, try to assign a smaller to parameter data_backup_buffer_size in global.ini.
In our case, we have enough hard disk space for the restore... We observe the memory and disk utilization (using df and top commands) while the backup restore is being performed. Disk is maximum 60% used, however I see how RAM gets consumed up to 98% at the time the backup recovery crashes...