Richfaces 3.3.3.Final
JSF 1.2
Under some circumstances (in our case it is a page change, a download of a document and some other stuff), we did not figure out whats the exact problem, the javascript call Richfaces.showModalPanel() is not working anymore in IE 11. No problem with IE 10, Firefox or Chrome.
Usually you upgrade the JSF and Richfaces version to something thats maintained and forget this old stuff problems, but this was not possible for other reasons.
The solution we found is to change the code from
oncomplete="if(#{!list.errorMessages}){Richfaces.showModalPanel('myPanel');}jQuery.unblockUI();"to
oncomplete="if(#{!list.errorMessages}){#{rich:component('myPanel')}.show()};jQuery.unblockUI();"
Now the ModalPanel is working in IE 11.