[Radek]

I know about two possible causes of the problem:
1. Some actions performed by the game take a long time. Until the action is completed, you cannot save the game or rest. Example:
If you send the sewer golem to cleanse the sewer in the Irenicus Dungeon, the golem will go away and "cleanse the sewer" and - you cannot save the game or rest until the golem returns. If this is the case, then you have triggered some long duration action or you have triggered some action that can not be finisthed for some reason. Backtrack the latest actions and see whether something need not be done to finish some pending action.
2. The misunderstanding in the scripts as mentioned by Olorin. Let us see what is happening in the Keep. The bug reveals itself if you start speaking with the guard.
a) You start a dialog with the guard.
b) The lady interrupts the guard and starts her dialog.
c) The dialog with the lady ends and BOTH the lady and the guard leave the scene - with the unfinished dialog with the guard.
You cannot save game and you cannot rest because you are still "speaking" with the guard. If you check the icons of your party members you will see a "talking icon" on one of them. Remedy: leave the keep and seek the guard outside the keep. "Talk" with him. This finishes the dialog with the guard. If this is the case then you should see a "talking icon" on some of your party members. In this case, backtrack you latest meetings with NPCs and finish the pending dialog. In fact, it is a special case of (1) - an unfinished action.