Access forbidden just when running update

I'm accessing an external repository linked via svn:externals, say
this external URL is
https://subversion.example.com/rootpath/myproj

When I checkout a dir from my own repository, the external reference
is retrieved fine.

If I change the svn:externals to another revision (the path is the
same) and run "svn update", I get this error:

svn: warning: Server sent unexpected return value (403 Forbidden) in
response to OPTIONS request for 'https://subversion.example.com/
rootpath'

But if I delete the external directory and update again, everything
comes fine.

I really don't have permission to this root path, but I don't
understand why it is necessary for an update and not necessary for an
complete checkout.

The client is version 1.6.9 and the server is version 1.6.3
Received on 2010-05-28 01:33:03 CEST