REST Web Service folders seem empty, URLs still have correct layer listed

[Hiportes]

Using ArcMap 10.5.1 - have a partner that we use their REST services. Had been no problem until recently. We can still connect to their REST endpoint, but the folders within it show up as blank (has the plus next to it, then the plus goes away and nothing show up within the folder). 

When using the endpoints in a browser, the layers still show up, and are listed as usual. They recently updated to https (those URLs are the ones that work in browser), but I am unsure if that has had any impact.

Any help is appreciated!

[DavidPike]

They may have also secured their services, or were they already secured?

I'm guessing you've attempted to re-add the connection?

What type of services are they, WMS, ArcGIS etc. ?

[Hiportes]

Thanks, for the response, David! 

It is an ArcGIS Server.

They did recently change from http to https - would this restrict us from seeing the data? If so, can they switch to https and also allow the data to be publicly available?

Thanks!