Posting from my phone, so I apologize in advance for any orthographical issues.
The encountered error message was due to a discrepancy between certificates & user accounts. I did not have the foresight to recommend reinstallation as I've always found myself defaulting to the true Admin account on my home computer, so I apologize if you were understandably confused.
Most importantly, if Mumble is now working even on your old account, I need to know if you had to change your User Account Control permissions as iterated in Step 1. Did you have to set the slider to "Never notify?"
In essence, all the true Admin account is doing is assigning itself full control over all folders & files, negating all potential permission issues, which shouldn't wholly be necessary for the overlay to work, but it has saved me from a lot of potential hitches. If you did have to change your User Account Control setting, then all you need to do is log back in to your original account & in the Command Prompt, without quotations, type in "net user administrator /active:no".
Otherwise, there are two ways to tackle this:
a. Right-click on the parent folder of your affected program, click on Properties -- I apologize for any potential discrepancies as I'm not at my computer -- and then I believe you click on the Permissions tab. Under Advanced, directly assign your User Account with ownership & Full Control.
b. Transfer all documents & AppData from your old account to the new true Admin account. You can then open "netplwiz" and automatically log on to the true Admin account
It's entirely possible that reinstalling Mumble on the true Admin account automatically elevated permissions of Mumble's folders & files to a system-wide Full Control, which is a big reason to use the true Admin account.