I've seen this I think. Not sure if same issue.
Not often...but sometimes the designer and the forum stalls.
I thought the designer and forum were on different servers though.
With the designer... I've had a few instances where I cannot select the preview option. I.e.: pressing it does nothing... but I can access all the other features - including publishing and exiting. Only preview seems to be doing this.
A couple time - after not being able to select the preview option.. . I waited for several minutes before trying again - and still was not able to preview. I had to exit and re-enter... however - I could publish.