No CSS on panel_content edit page

are there any errors in your apache/php/watchdog logs?

I too have got something like this.
I created panel_page in D6.13 and upgraded to 6.14.
It has content created within panels (and usin panels menu) inside the panel in specific area in right column.
Now - when I click "settings" and edit the body of this content and I click finish - there's some error.
However my change is saved, screen goes white with just one line of text:
[ { "command": "replace", "selector": "#panel-pane-93", "data": "\x3cdiv class=\"panel-pane\" id=\"panel-pane-93\"\x3e\x3cdiv class=\"grabber\"\x3e\x3cspan class=\"buttons\"\x3e\x3cdiv class=\"ctools-dropdown-no-js ctools-dropdown\" id=\"ctools-dropdown-1\"\x3e\x3cdiv class=\"ctools-dropdown-link-wrapper\"\x3e\x3ca href=\"#\" class=\"ctools-dropdown-link ctools-dropdown-image-link\"\x3e\x3cimg src=\"/sites/all/modules/panels/images/icon-configure.png\" alt=\"\" title=\"\" width=\"16\" height=\"16\" /\x3e\x3c/a\x3e\x3c/div\x3e\x3cdiv class=\"ctools-dropdown-container-wrapper\"\x3e\x3cdiv class=\"ctools-dropdown-container\"\x3e\x3cul class=\"links\"\x3e\x3cli class=\"0 first\"\x3e\x3ca href=\"/?q=panels/ajax/hide/panel_context%3Apage-pokojgoscinny%3Apage_pokojgoscinny_panel_context/93\" class=\"ctools-use-ajax\"\x3eDisable this pane\x3c/a\x3e\x3c/li\x3e\n\x3cli class=\"1\"\x3e\x3ca href=\"/?q=panels/ajax/configure/panel_context%3Apage-pokojgoscinny%3Apage_pokojgoscinny_panel_context/93\" class=\"ctools-use-modal\"\x3eUstawienia\x3c/a\x3e\x3c/li\x3e\n\x3cli class=\"2\"\x3e\x3ca href=\"/?q=panels/ajax/style-type/pane/panel_context%3Apage-pokojgoscinny%3Apage_pokojgoscinny_panel_context/93\" class=\"ctools-use-modal\"\x3eStyle: No style\x3c/a\x3e\x3c/li\x3e\n\x3cli class=\"3\"\x3e\x3ca href=\"/?q=panels/ajax/pane-css/panel_context%3Apage-pokojgoscinny%3Apage_pokojgoscinny_panel_context/93\" class=\"ctools-use-modal\"\x3eCSS properties\x3c/a\x3e\x3c/li\x3e\n\x3cli class=\"4\"\x3e\x3ca href=\"/?q=panels/ajax/access-settings/panel_context%3Apage-pokojgoscinny%3Apage_pokojgoscinny_panel_context/93\" class=\"ctools-use-modal\"\x3eVisibility settings\x3c/a\x3e\x3c/li\x3e\n\x3cli class=\"5\"\x3e\x3ca href=\"/?q=panels/ajax/access-add/panel_context%3Apage-pokojgoscinny%3Apage_pokojgoscinny_panel_context/93\" class=\"ctools-use-modal\"\x3e -- Add new visibility rule\x3c/a\x3e\x3c/li\x3e\n\x3cli class=\"6\"\x3e\x3ca href=\"/?q=panels/ajax/cache-method/panel_context%3Apage-pokojgoscinny%3Apage_pokojgoscinny_panel_context/93\" class=\"ctools-use-modal\"\x3eCache method: No caching\x3c/a\x3e\x3c/li\x3e\n\x3cli class=\"7 last\"\x3e\x3ca href=\"/?q=%23\" class=\"pane-delete\" id=\"pane-delete-panel-pane-93\"\x3eUsuń\x3c/a\x3e\x3c/li\x3e\n\x3c/ul\x3e\x3c/div\x3e\x3c/div\x3e\x3c/div\x3e\x3c/span\x3e\x3cspan class=\"text\"\x3eWłasny\x3c/span\x3e\x3c/div\x3e\x3cdiv class=\"panel-pane-collapsible\"\x3e\x3cdiv class=\"pane-title\"\x3eBrak tytułu\x3c/div\x3e\x3cdiv class=\"pane-content\"\x3e\x3cp\x3e\x3cp\x3e Za granicą \x3c/p\x3e\n\x3c/p\x3e\x3c/div\x3e\x3c/div\x3e\x3c/div\x3e" }, { "command": "changed", "selector": "#panel-pane-93", "star": "div.grabber span.text" }, { "command": "modal_dismiss" } ]
I attach two images - from body editing and error white screen.

When you get the above message, is there a javascript error? (in firefox hit ctrl-shift-j to bring up the javascript console. Hit clear to remove any errors from other stuff, then click the button that brings up the error).

Yes there is an error, but I don't see nothing common with panels.:

Error: Drupal.settings.img_assist is undefined
Source file: http://localhost/sites/all/modules/img_assist/img_assist.js?V
Verse: 15

I believe there is an issue in the img_assist queue about this already. I don't have the nid handy, however, so you're going to have to search for it.

For some reason the CSS for the panel content editing area is not working for me and I can't edit the placement or anything for the things on paneled pages. The site has been moved around lately since we're moving to a new server, so I'm thinking that the likely culprit is that the CSS file that controls this page got lost. Where should I look to replace it? I've tried reinstalling Ctools after getting an error message pertaining to that for this page, and I've reinstalled the Panels module, but to no avail.