Apologies for the late response, the project was temporarily put on hold.
I believe that MAI is a dependency that was used before I took over this project and is no longer used. That error seems to be popping up randomly and not actually affecting the site.
I was able to find out that the missing resource files were due to a broken DLL file that was supposed to make the site mobile friendly. By removing this module from my web.config I was able to fix the site.
I made the finaly upgrade to 9.8 and the site is mostly working, however now I am getting a strange error when I try to login:
System.NullReferenceException: Object reference not set to an instance of an object.
at DotNetNuke.Security.Membership.AspNetMembershipProvider.UserLogin(Int32 portalId, String username, String password, String authType, String verificationCode, UserLoginStatus& loginStatus)
at DotNetNuke.Entities.Users.UserController.ValidateUser(Int32 portalId, String username, String password, String authType, String verificationCode, String portalName, String ip, UserLoginStatus& loginStatus)
at DotNetNuke.Modules.Admin.Authentication.DNN.Login.OnLoginClick(Object sender, EventArgs e)
at System.Web.UI.WebControls.LinkButton.OnClick(EventArgs e)
at System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String eventArgument)
I found this page with a similar issue: https://www.dnnsoftware.com/forums/threadid/240951/scope/posts/nullrefexcept-dotnetnukesecuritymembershipaspnetmembershipproviderupdateusermembership-userinfo
I'm not sure how to proceed from here. Which libraries should I try to remove? And should I be replacing them with something? Should I try running the DNN_9.8_Install package?