How do I debug a failing ember-cli build?

I have an oldish ember-cli project. It's Ember 3.5, but it came from 2.x initially (and has some bower components). It used to build and deploy fine. But now I started it from a clean computer. I changed nothing, except reinstall its dependencies.

npm install
./node_modules/bower/bin/bower install
./node_modules/ember-cli/bin/ember build -prod

After some processing, I see the following error:

ajv.js: property right of assignmentexpression expected node to be of a type ["expression"] but instead got null

I try to lock some versions (i.e. remove ~ and ^ from package.json version numbers and run npm install again) and I either get the same error or something equally vague.

async.js: Property object of MemberExpression expected node

Trying the same with bower results in other error messages:

uglifier: Unexpected token: punc (:)

There were others but I didn't write everything down. I've spend quite some time on this. The problem is that every error, including their stack trace, doesn't really help me find where the problem is.

How can I debug this? How can I fix this? I thought package-lock.json was made to prevent this, but I guess I'm in dependency hell either way. Perhaps it was a problem with bower.json? There is no bower-lock.json.