Can't navigate anywhere past the dashboard in backend

I'm a new Sitefinity developer taking over my company's website. There's quite a few issues that my marketing department wanted addressed, so in order to make some changes I set up a development server. This involved copying all the code from our existing internal staging server, as well as pulling the most recent database from our production server. I've followed the documentation quite meticulously for how to host projects on IIS, and several errors later I managed to get the website up and running. 

However, after logging into the Sitefinity backend, I started to notice some issues that I hadn't a clue where to begin scoping down where the problem lies. When I try to navigate past the dashboard, I get three weird errors, depending on the page I'm attempting to hit. I either get a Page Not Found error (for example, on the Advanced Settings page). Of which, the console throws this:

Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=manager1_TSM&compress=1&_TSM_CombinedScripts_=%3b%…:73 Uncaught Error: Mismatched anonymous define() module: function ()return Z
requirejs.org/.../errors.html
    at B (Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=manager1_TSM&compress=1&_TSM_CombinedScripts_=%3b%…:73)
    at M (Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=manager1_TSM&compress=1&_TSM_CombinedScripts_=%3b%…:80)
    at Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=manager1_TSM&compress=1&_TSM_CombinedScripts_=%3b%…:91
Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=manager1_TSM&compress=1&_TSM_CombinedScripts_=%3b%…:73 Uncaught Error: Mismatched anonymous define() module: function ()"use strict";return Swiper
requirejs.org/.../errors.html
    at B (Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=manager1_TSM&compress=1&_TSM_CombinedScripts_=%3b%…:73)
    at M (Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=manager1_TSM&compress=1&_TSM_CombinedScripts_=%3b%…:80)
    at Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=manager1_TSM&compress=1&_TSM_CombinedScripts_=%3b%…:91
api.demandbase.com/.../ip.json Failed to load resource: net::ERR_NAME_NOT_RESOLVED
dev.securestate.com/page-not-found Failed to load resource: the server responded with a status of 404 (Not Found)

On other pages, for example on the Forms page, I get a Javascript alert: 'Could not deserialize service response, because it was empty or not valid JSON'

Then, if it isn't one of these two defined errors, the page just comes up blank.

I'm not quite sure how to proceed from here, though something major is wrong. Could you give me some ideas of how to track this down?

I'd really appreciate any help. Thank you!