The error you are getting indicates some kind of problem initializing the "Grails classpath container". This is how STS configures the internal Groovy compiler's classpath.
All of the other problems are most likely a result of the classpath container problem.
The question is... why is the classpath container broken? We have to figure that out.
Can you try doing "Grails Tools >> Refresh Dependencies". This will force an update of the classpath container. In the version you have, errors from that operation will only get logged in the error log (I only yesterday put in a little fix that will make errors from the refresh dependcies popup in a message box shown to the users, which may hopefully help in the future to know when/why something isn't working).
So after trying that can you check the error log (Go to "Window >> Show View >> Other >> Error Log") and see if there are any errors? Post what you find here.