vt7: Unable to open the Mail Converter from the menu

In CRM Settings, got to Other Settings, click on Mail Converter. I get a blank screen and the following error in the browser console:

The character encoding of the HTML document was not declared. The document will render with garbled text in some browser configurations if the document contains characters from outside the US-ASCII range. The character encoding of the page must be declared in the document or in the transfer protocol.

I guess this is probably related to some of the installation errors mentioned in #534 (closed)