I just faced a problem just re-opening an old project in Domino Designer - here version 10. I remember that I used the same project on the same DDE a week ago without issues. What I got now was this in the "Problems" tab of DDE:
My first thought was: was it really the same DDE like last week and maybe this installation lacks of all the extension this project uses?
No, it's much simpler. For whatever reason a setting in DDE's Java compiler options cracked in that didn't a week ago. I won't examine why this didn't happen last time but for now I am able to build the project again after re-setting that option to a reasonable value.
In DDE preferences, goto "Java, Compiler, Errors/Warnings" and search for "forbidden" in the search box on the top of the properties page. What you want to set is the "Fobidden reference (access rules)" value to "Warning" instead of "Error".
Again, I am not sure why this tuned in this time but whenever you get this error this doesn't necessarily mean that you did something wrong in your coding.