Eclipse error - "The project needs to migrate WTP metadata"

It got this error "The project needs to migrate WTP metadata" after importing a project from SVN. A working project.

So after trying various things, including:
right button (in the error) - Quick Fix,
restarting Eclipse,
opening and closing the project,
etc...

the solution that worked was:
Rename the project, and then rename it back.

No comment.