ReportViewer 2005 control and Soap versions

We have just upgraded out systems to use SQL 2005 and the new
ReportViewer control within our applications.
Whilst this works OK on most sites, one or two cannot render any
reports, but get the cryptic message
"Client found response content type of 'text/html; charset=utf-8', but
expected 'text/xml'"
and then shows some well formed XML beneath it.
They are all running the same versions of the applications, to the same
single report server, but some have to go through different firewalls.
They can all reach our servers as they can all make web service calls
sucessfully.
I think that something is stripping out the SOAP wrapper from around
the request/response but I cannot get any futher.
Does anyone know what version of SOAP the underlying RS communication
is using - I think the old viewer used SOAP 1.1 - and I thought that
the new control used SOAP vserson 1.2As an additional request....
Has anyone been able to turn IIS tracing on for the ReportServer
directory so you can use TRACE.AXD to see what message data is going
too and from the reportserver site ?
ReportServers own logs, even on verbose, only show what is is doing
after it has parsed the request, and the IIS logs only show a request
being made, not the request contents.
When I try \\localhost\ReportServer\trace.axd it just shows the
subdirectory structure and not the normal tracing web page I would have
expected.