@JoMa1 just solved this problem updating the firmware to the 01.07 version.
You cand find the download here -> PD.01.07
After updating the firmware, on the configuration files option under the maintenance menu, you have to select the running conguration file on the source and the backup file on the destiny. After this process, the current configuration that is running on the switch will be copied to the backup configuration file. Then you can download it, it won't show you the error anymore. Works just fine!
Regards