OK, so I loaded this in IE, and it throws a hopefully useful error.

Error loading "/Providers/HtmlEditorProviders/Fck/FCKStyles.aspx?tabid=248"

Follow by more:

I figure I can probably fix this with an appropriate regex expression. Can you point me in the right direction? I tried a few things, but they didn't work.

OK another customer provided me with a Fiddler trace, which showed an unwanted 301 redirect on the FCKStyles.aspx Url.

I can't recreate this in testing on my install of 4.9.0 and UM of 1.12, but it clearly exists, perhaps with an earlier version of UM.

Here's what I recommend :
- in the 'advanced regex filters' of the 'Friendly Url Settings' page, add the following values.  If you already have something in the text box, just append the value to the end of the existing regex expression

doNotRedirectRegex : /HtmlEditorProviders/  (if existing value, append |/HtmlEditorProviders/ to the end of the existing expression)

doNotRewriteRegex : /HtmlEditorProviders/ (if existing value, append |/HtmlEditorProviders/ to the end of the existing expression)

This should stop any unwanted rewrite or redirects of the FCK components.

Please let me know if this works.

-Bruce