Greetings,
I'm running into the same issue here but clearing out all the .nxt files and relaunching the client didn't seem to resolve it... That said, I'm not sure how to close out the NXT client that's running so it could simply be that I need to do that first..
I tried ending the Java binary process that I saw running, but it seems all that did is killed the GUI on port 7875, but now I see a GUI on port 7874 without any blocks listed etc.. Odd thing is that 7874 used to give me an error message..
Another weird thing I'm running into is that I cannot pull up my client at http://localhost:7875/ -- I always get a 404. To access the GUI I have to use HTTPS and then it gives me a certificate error (that I can bypass).
Working on Windows 8..