Can't ssh into my FB 4040 any more

Hello,

a few days ago I updated my FB 4040 to the latest version of OpenWRT 21.02.1. Today I wanted to downgrade from "bash" to "sh" again, which I had previously installed for convenience. So I changed /etc/passwd from

root:x:0:0:root:/root:/bin/bash

back to to

root:x:0:0:root:/root:/bin/sh

After that I couldn't ssh into the box any more, I was told

"Permission denied (publickey)"

Which seems odd, because I only changed the user's shell. After that I tried

root:x:0:0:root:/root:/bin/ash

and suddenly ssh works again.

Both shells are linked to busybox:

lrwxrwxrwx    1 root     root             7 Oct 24 11:01 /bin/sh -> busybox*
lrwxrwxrwx    1 root     root             7 Oct 24 11:01 /bin/ash -> busybox*

What causes this strange behaviour? Is /bin/sh not supposed to be the default shell?

Thanks in advance for any help and clues!