After installation of m2eclipse plugin Eclipse was not able to resolve the absolute path of the jars in classpath.
The value of the M2_REPO variable was changed itself to some path other than what I configured. When I tried to fix that, to my surprise I can’t edit it any longer. It was marked with this text “M2_REPO (non modifiable)”.
i experienced the same as this guy, but my solution was not quite the same.
i got eclipse to use an external maven’s settings.xml (eclipse preferences -> maven -> user settings), which had <localrepository> set to the right location.