cannot edit pages

Hi,
a few days ago I installed my first WordPress domain. Almost everything was ok. I created some pages, downloaded pictures, and wanted to add SEO plugin. Here the problem started. I couldn’t turn on the plugin and use it. There was “500 Internal Server Error”. My server helpdesk suggested changing settings in PHP. I changed it to 5.3 and turned on Magic Quotes function. The pluggin was fixed, but I couldn’t edit any pages. I reinstalled WordPress, cleaned cache, changed theme, turned off plugins. It’s still wrong. I cannot put anything in writing window.
I will be grateful for any help.