I found a solution! Please check the first answer here - it worked perfectly.

I'll leave the issue below in case this will be able to help anyone else with a similar issue.

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Hey all. I apologize to post a tech support question like this, but I've read through a fair number of posts about this in various locations already, yet I haven't seen anyone actually resolve this issue (most were regarding Windows 10), plus I'm in a weird spot in that it was working the first day I got it. I'm on Windows 7 x64 Ultimate, and nothing has changed since then except for powering down my PC each night.

Any time I go to try and access it I get this error. Trying to use Diagnose does not solve it, but it does return this in the troubleshooting. On the 3DS's end, everything seems to be completely normal, and other wireless features such as the eShop work completely fine. It reports no errors and connects to the network without an issue.

The most confusing part of this is the fact that it was working until I powered down my computer that first day. I've tried restarting my router, starting my connection settings from scratch on my PC and 3DS, disabling my firewall and anti-virus, mapping a network drive, but nothing is working. It's driving me absolutely mad.

Is there anything else that anyone can suggest to get this working again? I will contact Nintendo as well but since this seems like more of a problem with Windows, I figured it wouldn't hurt to message here as well.