No error appears in the console and no PHP notice in the HTML.
When I tried updating to 3.0.x branch, it did not change the behavior.
I can now replicate the problem also in items admin dashboard by clicking e.g. "Revised" column (to sort by revised. See attached screenshot of the "broken" admin layouts.
I can see that a whole lot of flexi_xxx.css and flexi-libxxxx.js is not loaded when the layout breaks. The HTML source of the healthy page looks like this:
while the HTML source of the broken page looks like this (notice the empty gap where there are normally script references?):
Code:
<link rel="stylesheet" href="/A.administrator,,_components,,_com_flexicontent,,_assets,,_css,,_flexicontentbackend.css,,q7354d0787c8f2b309d3f18c072ecf170+administrator,,_components,,_com_flexicontent,,_assets,,_css,,_j3x.css,,q7354d0787c8f2b309d3f18c072ecf170+components,,_com_flexicontent,,_librairies,,_jquery,,_css,,_ui-lightness,,_jquery-ui-1.9.2.css+components,,_com_flexicontent,,_librairies,,_select2,,_select2.css,,q3.5.4+components,,_com_flexicontent,,_assets,,_css,,_tabber.css,,q7354d0787c8f2b309d3f18c072ecf170+media,,_system,,_css,,_modal.css+administrator,,_templates,,_isis,,_css,,_template.css,,qcdf901f132ed2b0968e7ee239e8e94e5,Mcc.rFGPBj0XEG.css.pagespeed.cf.zdvDLpWd7f.css" type="text/css"/>
<style type="text/css">.mce-in{padding:5px 10px!important}div.pane-sliders ul.adminformlist li select{margin-bottom:0px}div.pane-sliders ul.adminformlist li fieldset{margin:0;padding:0}div.current ul.config-option-list li .fcsep_level3{left:232px!important}div.control-group div.control-label label.hasTooltip,div.current ul.config-option-list li label.hasTooltip{display:inline-block;padding:4px;margin:1px 6px 0px 1px;text-align:right;width:220px;font-weight:normal;font-size:12px;background-color:#f6f6f6;border-bottom:1px solid #e9e9e9;border-right:1px solid #e9e9e9;color:#666}div fieldset select{margin:0px}div.current ul.config-option-list li select{margin-bottom:0px;font-size:12px}div.current ul.config-option-list li fieldset{margin:0;padding:0}.tool-tip{}.tip-title{}.menu-akeeba-16{background:url(http://52.63.186.147/media/com_akeeba/icons/akeeba-16.png.pagespeed.ce.wP2TkGbDyB.png) no-repeat}.menu-flexicontent{background:url(components/com_flexicontent/assets/images/xflexicontent.png.pagespeed.ic.9eZcccbh4a.png) no-repeat}</style>
<script src="/media/system/js/core.js.pagespeed.jm.j38qmNNAsD.js" type="text/javascript"></script>
If you would be able to provide suggestions for next troubleshooting steps that would be awesome.