Forum OpenACS Q&A: Re: Fatal: received fatal signal 11 - new error after years!
Looks like a couple of things are happening.
First, there's a query error related to a scheduled notification, probably repeatedly trying since it is failing..
Second, there may be a backlog of scheduled events that are using up available memory and causing the signal 11 (just a guess).
You'll want to fix the query error. Its cause appears to be two users with the same email address, where the query expects users to have unique emails. (I edited out the particular email that was posted here, but you can see it in your error.log version).