bug#35473: accessing remote openwrt hosts gives => Wrong method specification for `ssh'

[emacsbugs . grokchem]

In emacs on a Debian stable desktop:

  C-x C-f /ssh:myopenwrt-host:/etc/config/

remotely opens the directory on the openwrt router just fine. When a
file in that directory (e.g. "system") is selected, emacs gives:

  "Wrong method specification for `ssh'"

If the same file is selected a second time, emacs opens an empty
buffer.

There is no problem accessing these files without emacs using ssh and
scp.  FWIW, this is the relevant ssh config:

  host myopenwrt-host
    hostname          192.168.1.38
    port              22
    user              root
    IdentityFile      /home/user/.ssh/id_rsa
    ForwardX11        no

* versions *

GNU Emacs 24.5.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
OpenSSH_7.4p1 Debian-10+deb9u6, OpenSSL 1.0.2r  26 Feb 2019
openwrt-18.06.2-brcm47xx-mips74k-asus-rt-n16-squashfs.trx