In Eclipse, what can cause Package Explorer "red-x" error-icon when all Java sources compile without errors?

yeah, this happens sometimes for no apparent reason. You can go to the "Problems"-Tab (right next to console output) and see the error message, so maybe you can narrow it down that way.

This happens often when I use Maven, and I had always ignored it until I found this question. You need to update the project in this case (figured this out by looking in the Problems pane)

From the project context menu: Maven -> Update Project and select the available maven codebases

Alternatively you can use (Alt + F5) to bring up the same window

Try to clean the project and rebuild it.