1 -Try to create new share folder, and new user and fgive full permissions to that folder (including Guess access rights.
2 – Change “strict allocate = yes” to “strict allocate = no” in smb.conf file. (its under /etc/config/smb.conf. You can download winscp & connect your NAS as admin to access..
3 – Try Reseting system configration to your NAS (I suspect your network configrayion, maybe Ip adress, wrong subnet, wrong Load balance maybe..)