Browservorschau funktioniert seit Tahoe 26.0.1 nicht mehr

Is the local site folder defined and residing on your primary hard drive?

What do you see when you go to 

File > Open in Browser?

RealTime Preview is useless. I disabled it in my Preferences settings. See screenshot.

I dont use the testserver.
I use Safari and Chrome to browse:
I've already removed both from the list and added them again.

Is the testing server independent of both computers?  Live view/browser preview works utilizing your testing server so if the server is a localhost both computers need an identical localhost setup, otherwise, if the server is remote it's possible one computer isn't connected causing live view to not display.