Creating new Sighting 500 error

I think the problem is caused by x-select. After version 3.0.0, x-select has some problems assigning the value in template directly to the model. Maybe you can try what tgandee said, use action instead of value to assign the value: Install EmberX-Select p.437, just like this:

// new.hbs
{{#x-select on-change=(action 'didMakeCryptidSelection')
            class="form-control" as |xs|}}
    {{#xs.option disabled=true}}Select Cryptid{{/xs.option}}
    {{#each model.cryptids as |cryptid|}}
        {{#xs.option value=cryptid}}{{}}{{/xs.option}}

// controllers/sightings/new.js
actions: {
    didMakeCryptidSelection(value) {
        this.get('sighting').set('cryptid', value);

If you still have any problem, maybe you can have a look at what I did.
And sorry for my poor English; English is not my native language:-)