• The problem seems to be specific to child portals.
• The problem was not always present; the problem just appeared a couple of days ago and I'm not sure of what could have caused it.
• The problem may be inconsistent.  The problem was first reported to me yesterday, so I created a test account and was able to successfully consistently reproduce the problem.  Today, however, the problem persists for the original user that reported the issue, but I'm no longer able to reproduce the error.
• An administrator can workaround the error if they explicitely grant themselves edit access to the particular instance of the Text/HTML module instead of relying on role membership.
• Unlike the original thread that discussed this issue, we are not using "localhost".  That gave me the impression that it might be related to cookies, but the problem did not go away after I cleared out my temporary internet files and cookies.
• Restarting IIS didn't help.
• I'm using DotNetNuke version 4.8.1.
• I tested via two different child portals on two different sites.  I was able to reproduce the problem reliably yesterday on both sites & servers but not I cannot reproduce the problem at all today.
• People with Host/Super-user privilege have not been effected (yet).  They can edit just fine.
• Creating a new special role other than relying on the Administrators role does not help.  Only granting access to individual users seems to help.
• Giving "Edit" access to the "All Users" role will make the problem go away too.
• This error only occurs via the "Edit Text" link when using the Rich Text (Cute Editor) editor.  The error does not occur when editing the content inline or when using the basic textbox editor.

I don't need a lot of attention on this issue because I have a halfway decent workaround, but I'm posting the issue nonetheless in case anyone has any advice.  Namely, can anyone think of some sort of cache somewhere that might be corrupt?