Please validate the following link from salesforce 403:denied_by_security_policy:create_denied - Salesforce Developer Community that talks about you might be subscribing to non existing event.
I have checked all these posts, the problem is with Informatica secure agents, No issues with connection configuration/events, once the connection unpublished and republished it is not refreshing to latest data and that could cause an issue, the same events and the same connection worked fine after several rounds of connection publishing and restarts of the process server.
Another strange thing is one event gives token Invalid error whereas another event works absolutely fine from the same salesforce connection.
there is no proper documentation or KB's available for Salesforce Topics/events from Informatica.
If the issue seems to be with the agent, then I think this will require deeper analysis and most likely a repro. I would suggest you open a ticket with Informatica support so that we can further look into it.
pls try re-starting the agent and see if there is a consistent behavior. If not raise a Case with support team.