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?

And I have some news about the situation. ...Still strange but after some work I have 'a service is already running on port ' exception during snapshot. But manually starting WAKomEncoded on that port is ok... And clicking "proceeding" also works.

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?

--
Dennis Schetinin