On the first, if there's some other persistent action, like sitting for example, you will be prevented from changing weapons or any other sort of equipment. In order to change, you have to cancel whatever action is in progress. Tapping one of the forward/backward keys usually does the trick.
And as Filatus said, the second is a HAK loading issue, which is solved by quitting all the way out of the game and coming back in (i.e. restarting the NWN program completely).