Cannot access some shares in Windows Explorer

I have created  on my NAS325 v2 three shares (S1, S2, S3), to be used by three users (U1, U2, U3). I don't remember what exactly I did when I created S1 and S2 some time ago, but when I look into the settings, it seems to be the same for all: The owner of all is "admin"; admin and U1 have full access to S1, admin and S2 full access to S2 and admin and U3 full access to S3. However, from the Windows explorer (Win10; after opening \\NAS325 I see all three S1, S2 and S3):
- I can access S1 from any computer; on my own directly, from others by giving user name U1 and the corresponding password
- I can access S2 from any computer without giving user name and password
- I cannot access S3 at all. After typing U3 and the corresponding password (same with "admin" and admin-password), I get an error message that I do not have the rights required to access the network resource.
Any one and idea what I do wrong and where the differences may come from? Thanks for any help - I don't have much experience with these things.