I am also having problems with not being able to select/interact with npcs using my mouse. I can click F to interact with them, and I can click just fine within the windows, however. Serynda’s description of the issue fits my problem exactly.
I was fine playing yesterday, also.
Edit: So I literally tabbed out, came to this forum and posted my issue, and then tabbed back into GW2 and I was able to hover over an NPC and it changed my mouse to a talk button. Weird! We will see how long it lasts for me…