Tried disabling that plugin and it didn’t help. So guess that one is not the culprit.
Plus the plugin was not there when the problem occurred earlier.
It is definitely related to some permalinks issue but I can’t pin it. I have seen this error crop after installing some plugin. For eg it cropped up after installing private msging plugin for forums. Disabled it, resaved the permalinks and rebuilt the forum indexes and the forums started working again.
But this time nothing is helping.