HTTP becomes HTTPS becomes HTTP...

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C#

.NET

MVC

WebAPI

.NET4.6

Hey all,
Perhaps this question is a bit weird... I'm doing my best to explain the issue.

So we're having a REST service (C# MVC Web API) running at http://something.com/api.
In another MVC web project we're doing some calls to the service pretty much as follows:

using (HttpClient client = new HttpClient())
{
    HttpResponseMessage response = client.GetAsync("some_uri").Result;
    if (response.IsSuccessStatusCode)
    {
        return response.Content.ReadAsAsync<T>().Result;
    }
    else
    {
        // Handle error...
    }
}

So far so good, everything works fine.

The URI is coming from the config file and, as said, is something like http://something.com/api.

All is well, until all of a sudden it isn't.
What happens is that the service becomes unreachable until I change the URI in the config to httpS://something.com/api.
And that works for a while until all of a sudden I must change it back to http (without the S)...

I'm thinking it must be a client side issue because last week I had this problem and had to switch to httpS while a coworker could continue working on http. Two of my coworkers are experiencing the same problem. There doesn't seem to be any obvious trigger that makes the change to HTTP(S).

Please don't tell me I should be using httpS all the time anyway, that's not helpful.
I've tried Googling, but all I can find is browser caching, HSTS and redirects, but those can't be the problem (obviously we're not using HSTS or redirects and I'm not sure how browser caching would cause this problem).

Any ideas on what could be causing this are appreciated.
Thanks!

What I have tried:

Googling this issue with no success.