this message occurred like "cannot read 20bytes at offset -20(:30): ([50] request not support)"

hello i'm haesu 

we are backup nas by data protector 7

this message occurred like "cannot read 20bytes at offset -20(:30): ([50] request not support)" during backup

but backup working is complete 

so i want know about this message 

and i was check filesystem about nas disk 

however disk status is good 

so can tell me about  this message?