Check the contents of the b4a file with a text editor.
I got very same error in strange situation.
My Laptop shut down unexpectedly and I could not open project after.
Strange thing is that for the same project I had no problems opening it on other PC
(its located in Dropbox so I have access)
then I found in .b4a file some extra lines:
ManifestCode='This code will be applied to the manifest file during compilation.~\n~'You do not need to modify it in most cases.~\n~'See this link for for more information: http://www.b4x.com/forum/showthread.php?p=78136~\n~AddManifestText(~\n~<uses-sdk android:minSdkVersion="4"/>~\n~<supports-screens android:largeScreens="true" ~\n~ android:normalScreens="true" ~\n~ android:smallScreens="true" ~\n~ android:anyDensity="true"/>)~\n~SetApplicationAttribute(android:icon, "@drawable/icon")~\n~SetApplicationAttribute(android:label, "$LABEL$")~\n~'End of default text.~\n~
Luckily after removing them everything was fine.
Maybe you should check how this could happen cause lot of people could have problem after they see they can not access their projects..