Queries on /data/ fail

Hi, I have run into a problem that I hope you can help me with.
I am building a web application that utilises your API and I try to query some data from the API.
I authenticate and log in just fine, and getting metadata also works.
However any query I try on the /data/ path results in a 500 Internal Server error returned. This goes for query, entityQuery, etc.

If I for instance retrieve /metadata/describeEntities on JobTitle I get a result just fine, but then running an entityQuery on JobTitle fails.

I understood that you have updated your service to only accept TLS 1.2+ recently, but according to a test query to https://www.howsmyssl.com/a/check my browser and code support up to TLS 1.3 so that should be alright, right?

Since I am developing on localhost there can't be an SSL certificate on my end but unless I understand the protocol wrong that shouldn't be required to issue a GET/POST request with authorization.

I am using AJAX in JavaScript to consult the API, example included below:

$.ajax({
type: "POST",
url: apiURL + "/data/entityQuery",
headers: {
    "Authorization": "Session " + sessionId
},
data: {
    typeName: "JobTitle",
},
success: function(res) {
    console.log(res);
}
});

Using a GET request yields the same result. What is going wrong?
I was expecting a normal error returned if I was messing up the syntax or anything, a 500 Internal has me at a loss.

Thanks!