[Radek]

1. This can happen not only if you are in the middle of a fight (according to the game) but also if the game is doing some long action. Example: If you put the sewer golem in the Irenicus' Dungeon into work then you cannot save/rest until the golem returns. Oh, well...
2. The lady in the de'Arnise Keep. It's a bug in the game. The bug consists in the following: The lady interrupded the guard's dialog and then both of them vanished - with an unfinished dialog (the guard's one). Now, the guard is still "speaking" and preventing you from saving/resting. If it happens to you then leave the keep through the secret door you have come in and go to a small camp nearby. The lady is there with her guard. Talk with the the guard. You need not tell him anything special - just start the dialog and quit it. Now, you'll be able to rest.
You can also reach the camp through the main gate if you have already opened it.

As far as your problem is concerned, the game seems to forget to clear some flag in its data. I can suggest only one thing: Change the map. Travel out of Waukeen's Promenade and try rest on another map. If it is a "hidden" enemy that follows you and prevents you from saving/resting, then the enemy should be removed from the game or teleported after you to a new map. In the first case, your problems are over. In the second case, you will see the enemy and be able to get rid of the problems. Perhaps...