Adobe Test & Target Extension Conflict
There seems to be a conflict with the Design Extender extension and Adobe's Test & Target extension.
I deleted the listed WA extensions and installed Design Extender. I had Abobe T&T already installed; when I opened an existing PHP page it loaded into the editor, stopped loading after a CSS Menu declaration and the page saved at that point, with no existing code beyond the CSS Menu declaration available.
I deleted DE, and deactivated all my extensions. DW worked normally. I then activated on extension at a time and tested. The extension that brought back the error was Adobe Test & Target.
Just a heads up -- I don't use T&T. Had installed it to try it out. . .
