Strange error message all of a sudden - Joomla! Forum - community, help and support

when trying save edited module, following:

not acceptable
an appropriate representation of requested resource /administrator/index2.php not found on server.

this happening on 2 joomla sites of mine, both on same server host...this leads me think problem related more setting (which haven't changed @ all, btw). ideas how can fix this?

could give more info locate problem:
what os (linux / windows )?
what webserver (apache / win iis)
what php version?
what mysql version?
what joomla version?

could check if necessary joomla files on server?
joomla diagnostics helpful tool check if joomla files have been uploaded correctly
http://extensions.joomla.org/component/ ... itemid,35/

furthermore similar topic might you: http://forum.joomla.org/index.php/topic,144508.0.html