Internal Server Error in every form

Hi i have a "Single Contact" menu item but when i click it is shows a HTTP 500 (Internal Server Error) instead of the form.
Also in the core Joomla Login module (or component) when the user inserts wrong data also a HTTP 500 Internal Server Error) shows instead what is is supposed to show.

In fact such errors show in every form (contact,register, login, virtuemart order number..)

I would really appreciate immediate help since i have posted similar tickets in the forum and the replies do not solve my problem.

Hi,
You can see some posts:
forum.joomla.org/viewtopic.php?t=344518
forum.joomla.org/viewtopic.php?t=680899

Also, please tell us:
1. Your site url. So we can see it.
2. Your PHP version?
3. Did you check the .htaccess file? Did your host support mod_rewrite?
4. Did you try rename these folders and then test?: /templates/ot_.../html/com_users, /templates/ot_.../html/com_k2, /templates/ot_.../html/com_contact, /templates/ot_.../html/mod_login.

5. Please make sure permission for all folders is set to 755 and files is set to 644. Just tmp or cache folder can be 777.


Thanks!

You can check it here (login) (error when user types wrong user or pass)
(contact)-- this does not appear at all

All you wrote is OK, but nothing happens, and there is no htaccess or any SEF URL settings applied.
I look forward for your reply

Anyone?

Can you assign template to default of Joomla! and check it ? so we can make sure that error appears from template or from server.

Regards

With default joomla template everything works fine (!!!!!)