Error: the XML response that was returned from the server is invalid

[gdinfo]

Hi all,
I have seen many posts about this subject in this forum but I have a strange situation:

My configuration is:
linux centos 4.6
xampp 1.6.2
cmsms 1.2.4 "Greenland"

I have 3 users: Administrator (Admin group), Editor1 and Editor2 (Editor group)

In the Admin interface, Content >> Pages, when trying to expand/collapse Pages & Subpages, Editor1 receives a popup whith the error when clicking on the triangle icon for expand/collapse pages. Doing the same operation as Editor2 (starting from the same client with same browser) or as Admin it does not present any error or message and all works fine!!!

Instead, the problem appear to all when clicking on the "Expand All Sections" below in the Page.

I have controlled unattended spaces at the beginning of some PHP files in the admin directory and in the lang (EN_US) one but all seems ok.

In the same server I have other 5 sites all with CMSMS 1.2.4 without problems.
Thanks in advance for any suggest.
Best Regards.

[gdinfo]

What described concerned IE 7.0.5.
Whith CROME and FF 3.0.3 it seems that there are no problems!!!!!!!
Best Regards