Open type (Ctrl-Shift-T) does not find types
Eclipse does not find certain classes that exist in the workspaces when Open Type (Ctrl-Shift-T) used. "Problematic" classes are from one certain directory. At the same time these classes can be found by Open Resource (Ctrl-Shift-R) and are perfectly compiled.
What settings should be changed to fix the problem?
To clear the Open Type indexes
Sometimes, the indexes that Eclipse use for searching the types, get corrupted. You can clear the Open Type indexes by going to <Workspace>/.metadata/.plugins/org.eclipse.jdt.core and deleting all files there. The indexes will be re-built the next time you use open type.
Note that this is a empirical observation, and you may delete some JDT saved data that you don't want to delete. I observed no ill effects, but for safety you could try deleting just the *.index files and the "savedIndexNames.txt", and see if it works.