Annoying Eclipse Validation Errors

Sometimes you run into these Unknown validation errors in Eclipse which you seem not to be able to get rid of.
The Maven build is running fine even from within Eclipse but the messages won't go away even if you:

• Right-click on the project --> Validate
• Project --> Clean
• Close and re-open it

What worked for me is: select them all, right-click and delete them.
If there was no real reason for them to show up because in the meantime you had removed the reason for them to exist, then they'll not be generated again.

Thanks to pelotom!