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.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.