Gustaf,
Just to follow up - Dave B and I just spent a bit of time trying to figure this out in IRC. Perhaps this will info will help:
Here's what appears at the beginning of a good request (i.e., the package is accessed directly, not through a host-node mapped site):
Notice: --starting... /intranet/wiki/en/index form vars = , ::43182 -> (14ms, 0ms)
Notice: --try en/index -> 0, ::43182 ::xowiki::Package->resolve_request (15ms, 1ms)
Notice: --try en:index -> 43223, ::43182 ::xowiki::Package->resolve_request (16ms, 0ms)
I'm simple typing in mainsite.com/mysite/wiki. Here's the request through a host-node mapped site that generates the error:
Notice: --starting... /wiki/ form vars = , ::262 -> (9ms, 0ms)
Notice: --try wiki/ -> 0, ::262 ::xowiki::Package->resolve_request (10ms, 1ms)
Notice: --try en:wiki/ -> 0, ::262 ::xowiki::Package->resolve_request (11ms, 1ms)
262 is the Main Site instance of acs-subsite and 43182 is the xowiki instance. It seems to us that in a request for the index page or the root url of the package, the invoke method sees /wiki as an object instead of "". The page error itself is generated by the invoke method when this code returns "": set page [my resolve_page [my set object] method].