I don’t know how, but when I inserted as username \ and left the password field blank suddenly it accepted the credentials and showed me the share contents.
Maybe it will help someone else who has the same issue.
EDIT: when I originally posted this answer I couldn’t check the checkbox to remember the credentials, causing me to have to do this on every boot.
However, this time it did allow me to check the checkbox, I don’t know what changed, but it’s working now. might have been a windows update.
Another Edit: Upon every boot I now have to use smbpasswd to re-configure the correct password for the account.
Final Edit: It seems this question is getting a lot of attention, I think I should add that after all this time, I managed to solve the issue.
This is related to 2 possible reasons:
- A complete factory-reset
- leaving the username as “admin”, I can’t recall exactly what or why, but changing it to my custom username resulted in issues, whereas leaving it as “admin” worked perfectly fine. Possibly something hardcoded in the NAS firmware.