I now get this error when trying to add files from my Mac or an external hard drive to my NAS. It started after I updated to Mac OS Big Sur. But this problem only happens on one mac. I have another mac that did not encounter this error and it was also updated to Big Sur. I reinstalled Big Sur but that did not fix the problem. I can copy files from my NAS to my local drive no problem. Everything crashes going the other way.
Any time I try to copy a file to one of my shares, it tries for a while and then the drive unmounts and I get the error. Then I am knocked off of any internet connection. I can only get it back after rebooting.