The problem is caused by the fact that some or all of the
files in your
html-template directory are marked as read-only. That should
be fine, but a
bug in beta 3 caused the error message you are seeing. This
bug has been
fixed internally, so it will be fixed in the final release.
As for what all those files are: They are all related to the
checkboxes in the project properties under "Flex Compiler"
Compiler"), in the "HTML wrapper" section.