As you probably know because you are searching for a solution to this error, the following Flex MXML code will not work:

<HBox visible="{((booleanValue1) && (booleanValue2))}" >

You will get this error message during compile:

Error: The entity name must immediately follow the ‘&’ in the entity reference.

Instead you will need to use html escape codes for these as follows:

<HBox visible="{((booleanValue1) &amp;&amp; (booleanValue2))}" >