This is the third time I got this message while accessing my forum. Yes, I only had to wait a few seconds to gain access but it does have me concern. Is something going on. Is it time for a bigger server? Are Nabble sites becoming very popular with increase traffic outstripping its ability to serve. I'm sure I'm not the only one being told by Nabble to wait...
From time to time over the last year I have also seen "Over Capacity" messages but, like you, by the time I have clicked through the message I then always get to where I want to go without further problem.
Whether it suggests that Nabble is becoming more popular or that traffic is now deliberately being restricted in some way to save costs I have no idea but, I agree, it is concerning for users of the service.
For me it happens so rarely that I have never made a note of the details, for example, if it is only affecting one server. I run multiple forums on some of their servers and my entire portfolio uses what I think is all bar one. None of them are high traffic, so there hasn't been a significant issue for my users yet.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me! GregHelp - Building a set of answers to Nabble FAQs.