Hey,

I've attempted to trace the problem. It happens inside
bookmark-default-handler when it calls (file-readable-p file). Therefore
the test case can be boiled down to:

(file-readable-p "/sshx:mbunkus@spector|sudo:root@spector:/etc/fstab")

This function call returns t if there's already a tramp connection open
with the very same host & proxy. It errors if there's no tramp
connection open yet.

Kind regards,
mosu