That part is interesting. There should never be aGET request to that endpoint as it only supports POST. I cannot find any place it the code that would call GET on that endpoint.
These should be removed from the log output, as they simply mention that a request from the UI was rejected, and there is no reason to show it without debug logging enabled.
When it crashes that way, it is the .NET runtime that crashes and that means that even the crashlog handler will never be invoked. The crashlog file that is mentioned may explain more, but most likely it will not be something that can be fixed in the Duplicati project.