Today I wiped my work notebook - new Windows, new everything. I installed Notes and DDE again of course. I installed also FP7 - no, not FP8 
I noticed a weird behaviour though: my Domino Designer wasn't able to find even the basic Java classes. This is a screenshot of it when I tried to built a project:
I little investigation brought me to check the installed JREs in the prefs of DDE:
For some reason the marked files in the next screenshot were not added to the JRE Domino Designer was using.
After adding them back to the JRE everything went fine.