Expected Behaviour:
- Operating System (Raspberry Pi OS. Raspbian ver. 13)
- Raspberry Pi 1 (Model A, IIRC, the one with the Ethernet port)
Trying to connect to the Web UI of the pihole on my PC. (Let's say 192.168.1.136)
Actual Behaviour:
Cannot connect with error "ERR_NETWORK_ACCESS_DENIED" from Chrome and similar on Firefox.
However I can still access the direct IP address of the Pihole from other devices that's not the PC. Checking my firewall rules on the router I don't think I have any exceptions involved that should be blocking access to it since mobile devices and even other PCs access it just fine.
Debug Token:
https://tricorder.pi-hole.net/yFuNivoo/
EDIT:
I should note that the Pihole is on a different VLAN from all the other devices on the network. However all allowed devices should be able to talk to the pihole just fine, just not the PC it should be also talking with as well.
In general your Router should not be involved in this unless we are talking about a Router that has more than 1 LAN attached to it 
I would suspect some kind of Firewall or Security Suite on your PC doing this...
The only thing I've done so far was to upgrade the network (I'm using Unifi gear with different VLANs) but it's only this particular PC with Windows 11 that's having troubles atm.
It's odd. I cleared the cookies and cache from the browser last time and it temporarily worked but when I started up the PC it doesn't work in this instance. Maybe I'll check Windows Firewall
Then it's related to that particular machine and its OS.
Check for stupid browser extensions.
Use a different browser. (Firefox instead of Chrome)
Tried on Firefox and Chrome. Even on a guest chrome browser (so clean browser with no extensions) and still getting to the same similar problem.
My Windows 11 PC had IPv6 enabled, and that allowed it to bypass my Pi-hole setup. Disabled IPv6 and it no longer bypassed Pi-hole. Not saying this is your problem, but a possibility.
This was a while back, and I cannot recall the steps I used to disable IPv6.
You mentioned that the PC is on a different VLAN from your Pi-hole setup. Is the PC on the same VLAN a the other devices? Depending on your router, you could also have a firewall on the router preventing the traffic from that VLAN, but if the PC is on the same VLAN as the other devices, this would not be the case.
I should note that my PC is on the same VLAN as Pi-hole. I suspect it will work, as I have network firewall rules setup to permit DNS traffic from other VLANs to the VLAN for Pi-hole. I may try the PC on one of the other VLANs to see if that works. I will update this response with the result when I can.
So the Pi-Hole is routed to a default VLAN.
I have all other devices routed to a "Trusted" VLAN like PCs, laptops, mobile devices. etc.
I have another "Untrusted" VLAN for IoT devices and such but shouldn't be communicating with the pihole outside of doing DNS requests.
Now it's just the one PC on the trusted VLAN that can't communicate with the pihole for some reason but everyone else can.
Now the PC I can't access directly to pihole, I can still remotely connect with Raspberry Pi Remote connect or what not but dunno how to see the stats like the web gui.
In your router, turn off IPv6 for the entire home network, and try again.
It is not a problem for me. My router handles blocking IPv6, but my prior router was a pain with doing that, so since my Windows box was the only one that supported IPv6, I handled the issue on that machine.
I added that as more of something to try on the PC to diagnose if that was in play.
Not at all familiar with Raspberry Pi Remote. After looking at it, seemed to me like overkill on a local network, and more aimed at access to a Raspberry Pi running their specific OS. That’s my take from a quick read. But I suspect the reason it works is because you are using something that goes from inside your network, out to some server, and back into your network.
Based on your description, I am assuming you have other devices that can use Pi-hole for DNS, including other Windows 11 devices, all working fine with Pi-hole.
Welp, I think I understand what happened.
I had a VPN client app (Mullvad) running on the PC that would start every time it ran.
I think it was trying to double connect on the VPN. One on the PC client side but also connecting the network side. So maybe having both active must have been causing some weird conflicts. It seems to be working fine atm, but I'll give it a few days to see again.
For now, I'll just have one active VPN running, lol