I don't know if it can be of any help, but I attach my smb.conf, as I noticed that in all the shares user andrea is both listed as a valid and unvalid user which, even if I'm not a samba expert looks me strange.
For some reason, Asuswrt has always done that as far as I can remember. I also have the same here. I suspect one of the two must have priority over the other one, so that quirk never really got noticed. Might be worth investigating just out of curiosity, however it's not the source of your problem because I have the same here on my USB thumbdrive.