cannot apply any change on views administration objects => apply does not work

Dear developpers,

the views interface is working correctly but, if I want to modify any parameter of a view, when I apply it, an ASCII text is displayed instead of the administration screen. Here is a copy of the text (the beginning)

[{"command":"settings","settings":{"basePath":"\/","pathPrefix":"","ajaxPageState":{"theme":"seven","theme_token":"k2Q351hG-FD7KwDg7kzgS3gVV82iXyt0cqmMTMztzTY"}},"merge":true},{"command":"viewsSetForm","output":"\u003Cdiv class=\u0022views-messages\u0022\u003E\u003Cdiv class=\u0022messages status\u0022\u003E\n\u003Ch2 class=\u0022element-invisible\u0022\u003EMessage d\u0027\u00e9tat\u003C\/h2\u003E\nSite en cours de maintenance. \u003Ca href=\u0022\/admin\/config\/development\/maintenance\u0022\u003ERemettre le site en ligne.\u003C\/a\u003E\u003C\/div\u003E\n\u003C\/div\u003E\u003Cform action=\u0022\/admin\/structure\/views\/ajax\/display\/matin_2013\/page\/style_plugin\u0022 method=\u0022post\u0022 id=\u0022views-ui-edit-display-form\u0022 accept-charset=\u0022UTF-8\u0022\u003E\u003Cdiv\u003E\u003Cdiv class=\u0022views-override clearfix container-inline\u0022\u003E\u003Cdiv class=\u0022form-item form-type-select form-item-override-dropdown\u0022\u003E\n \u003Clabel for=\u0022edit-override-dropdown\u0022\u003EFor \u003C\/label\u003E\n \u003Cselect id=\u0022edit-override-dropdown\u0022 name=\u0022override[dropdown]\u0022 class=\u0022form-select\u0022\u003E\u003Coption value=\u0022default\u0022\u003EAll displays\u003C\/option\u003E\u003Coption value=\u0022page\u0022\u003EThis page (override)\u003C\/option\u003E\u003C\/select\u003E\n\u003C\/div\u003E\n\u003C\/div\u003E\u003Cdiv class=\u0022scroll form-wrapper\u0022 id=\u0022edit-options\u0022\u003E\u003Cdiv class=\u0022form-item form-type-radios form-item-style-plugin\u0022\u003E\n \u003Cdiv id=\u0022edit-style-plugin\u0022 class=\u0022form-radios\u0022\u003E\u003Cdiv class=\u0022form-item form-type-radio form-item-style-plugin\u0022\u003E\n \u003Cinput type=\u0022radio\u0022 id=\u0022edit-style-plugin-grid\u0022 name=\u0022style_plugin\u0022 value=\u0022grid\u0022 class=\u0022form-radio\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-style-plugin-grid\u0022\u003EGrid \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-radio form-item-style-plugin\u0022\u003E\n \u003Cinput type=\u0022radio\u0022 id=\u0022edit-style-plugin-list\u0022 name=\u0022style_plugin\u0022 value=\u0022list\u0022 class=\u0022form-radio\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-style-plugin-list\u0022\u003EHTML list \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-radio form-item-style-plugin\u0022\u003E\n \u003Cinput type=\u0022radio\u0022 id=\u0022edit-style-plugin-jump-menu\u0022 name=\u0022style_plugin\u0022 value=\u0022jump_menu\u0022 class=\u0022form-radio\u0022 \/\u003E \u003Clabel class=\u0022option\u0022 for=\u0022edit-style-plugin-jump-menu\u0022\u003EJump menu \u003C\/label\u003E\n\n\u003C\/div\u003E\n\u003Cdiv class=\u0022form-item form-type-radio form-item-style-plugin\u0022\u003E\n \u003Cinput type=\u0022radio\u0022 id=\u0022edit-style-plugin-slideshow\u0022 name=\u0022style_plugin\u0022 value