File sharing always given authentication errors

I've used file sharing on all my Macs in the past, and it works great. I got a new M1 Mac Mini, and as usual, I tried to enable File Sharing on it.

Every time I attempt to connect to it from other Mac (e.g. my MacBook Pro), it gives an authentication error. Interestingly, it behaves differently when the password is wrong vs when it's correct, but disallows me in either case.

1. Attempting to connect via Finder with the correct username (`Alex`), but wrong password: it "shakes" the window, as expected.

2. Attempting to connect via Finder as guest (which should be allowed): it "shakes" the window

3. Attempting to connect via Finder using my AppleID: it "shakes" the window

4. Attempting to connect via Finder with the correct username/password: it doesn't shake the window, but gives this error dialogue box instead:

5. Attempting to connect over CLI with the correct password:

$ mount_smbfs //Alex@mini.home ~/Desktop/tmp/
Password for mini.home: <correct password>
mount_smbfs: server connection failed: Authentication error

6. Attempting to connect over CLI with the wrong password:

$ mount_smbfs //Alex@mini.home ~/Desktop/tmp/
Password for mini.home: <wrong password>
mount_smbfs: server rejected the connection: Authentication error # 

Interestingly, the error message is different.

Other info

1. Screen sharing works just fine, with the exact same username/password.

2. Here are my account's advanced settings:

3. Here's is the File Sharing portion of the Sharing preference pane:

And here's the page that opens when you click "Options..."

Troubleshooting steps I've tried

1. Restarting the (server) Mac.
2. Turning off sharing and turning back on.
3. Changing the account password (to the same value as before).
4. Checking the logs in `Console.app`, using search queries like `any: smb` and `any: share`. I couldn't find anything interesting.
5. I have also tried all the steps above using the "Full name" (`Alexander Momchilov`) instead of the "Account name" (`Alex`).

Any suggestions?