In versions earlier than DNN 9.2, the HTML Editor manager will load a tree view of all the pages in the Portal; therefore, if a site has hundreds of pages, the manager will take several minutes to load and it might even time out if it takes longer than the allowed execution time.
Site Administrator privileges.
The solution to this problem is to upgrade to DNN 9.2+ as the code was optimized to load the pages in a more efficient way.
Until the site can be upgraded, a possible workaround is to make the changes directly to the CKESettings table in the database. The setting names are usually the same structure of DNNCK + P for Portal, or T for tab + (#PortalID|TabID#) + Name. For example, for protected sources for Portal 0, the setting name will look like DNNCKP#0#ProtectedSource.
For further information, save the HTML Editor Settings and review the entries on your CKESettings table.