As posted in a similar thread:
You will get this error if there are any erroneous characters in any of the fields in the organization settings... so, for example, if the HTML code has gotten corrupt.
In my case, the text in the organization description, which had always been there and worked fine previously, no longer was acceptable for some reason.
THE SOLUTION is to check the HTML code yourself, or simply cut the text from all the description boxes, paste it out into Notepad (to remove any extraneous formatting), and bring it back in clean without formatting. You can then use the formatting tools provided if you'd like.
When writing descriptions of things in ArcGIS Online, whether for items you add, web maps you create or even your organization's description, it's best not to copy text from existing web pages. You get a lot of extra HTML code that you can't see. If you do want to copy text from a web page, paste it into Notepad first, which will strip off all HTML.
(Would be nice if the owner of the thread could mark this answered.)
