>> It looks like you are using a recent Pharo image and
>> Seaside2.8a1-lr.590. In this combination sometimes printing a walkback
>> causes another error. This is the stack trace you are seeing. Please
>> update to Seaside2.8a1-lr.591 to see the real cause of the problem.
>
> Are there instruction for update?

You just load the latest version through Monticello from the Seaside repository.

> Seems like port is still "busy" during or just after snapshot, but becomes
> "free" few moments later. Isn't it some kind of synchronization problem?

All sockets are closed during a snapshot, thus the web server has to
be restarted. I think I heard before that people run into problems
when setting the port environment variable and saving their images. I
don't remember the solution though.