Somehow the markup [[PageName|+]] doesn't work for some pages. It seems as if it doesn't work when page title contains special characters (like PmWikiFr.Sandbox with an accented character in (:title Bac à Sable:)).
Works now. There was a problem with PmWikiFr/Sandbox as Pm described at [pmwiki-users].
The problem is that the PmWikiFr/Sandbox page is set to
"auto-revert" (i.e., it's held somewhere other than wiki.d/ or
wikilib.d/ ), and auto-revert isn't enabled for the Test group,
so it's not seeing the page when done in Test.
I've now converted the page to be a "normal" one (no auto-revert),
and everything works as expected.