I was having the same problem. What finally worked was opening the Workspace files in a text editor and deleting any references to WA_Memento. Worked great!
Workspace files located on Mac at:
/Users/USERNAME/Library/Application Support/Adobe/Dreamweaver CS5.5/en_US/Configuration/Workspace/
There should be two files that need to be edited:
1. WORKSPACE_NAME.xml
2. WORKSPACENAME
Open them in a text editor and look for instances of "WA_Memento"
The tags that need removing should look something like:
<panelframe dock="floating" height="500" monitor="1" width="100" x="10" y="top">
<panelcontainer expanded="true" height="418" visible="false">
<panel height="418" id="WA_Memento" unconstrainedheight="418" unconstrainedwidth="669" width="669"/>
</panelcontainer>
</panelframe>
Delete these and you should be good!