File sharing/AFP skips "connect as"

I have two Macs, a Lion and a Yosemite. I have several accounts on each one, both user and admin. I activated file sharing on both Macs and selected AFP. I can connect from any account on one Mac to any account on the other one Mac as expected, with one exception.

"As expected" means: Select go->"connect to server" in the Finder menu, then enter one of the users names on the other Mac and its password, then choose which of he folders on the other Mac to mount.

The exception is that in one of the accounts the Yosemite Mac ("xyz" - unix name) I do not get the "connect as" prompt. It connects directly as the user "xyz" (same name) on the Lion Mac, without giving me a choice or asking for a password. It only asks me which folder I want to mount. The names are the same, but not the passwords. But so are all the other accounts, the account names on Yosemite mirror the account names on Lion.

This does not happen when connecting from xyz on Lion. It behaves as expected (see above). It does not automatically connect to xyz on Yosemite.

I also changed the password of xyz on Lion many times, and deleted the keychain, restarted, etc. Also, the Lion Mac is off-cloud.

What could be the cause of this behavior and how can I reset it?