default_value is defined as taking a string input in the API, the user shouldn't really be passing non-string values (arrays, hashes, ints, bools). It should probably be a string representation matching the UI. Or is some conversion happening prior to it reaching the model?

(Logs showing the issue may make this clearer.)

I agree that when it's defined as string, it should take only strings. Better error message would be appreciated though, but I'd prefer to handle it as a generic issue for the whole api.

I updated the description.
I agree with Tomas, Setting does allow to send an array as a value so this needs an error message or to change.

I'm running foreman 1.13.3-1. I'm having issues with overriding hashes (smart class parameter).. Yaml looks good but after I change the environment of the host I'll get a error 'value is invalid hash'. That happens to all the hashes which I want to override. It only happens after I try to change the environment of a host. Is this bug related to this? Many thanks!

Is your update through the api or UI? this bug is an api bug so if you're updating through the UI it's not related.
If it is unrelated please open a new issue with a description of steps and if possible attach screenshots or examples of the values.

thanks. It's indeed GUI related.
I'll open an new ticket.