Why can I SSH in to my Pi, but can't access the network from it?

Question

I recently re-arranged my home office, which required unplugging my router and Raspberry Pi 3 (among other things). Despite changing nothing besides the ethernet cable, I am now encountering the following problem.

I am able to SSH into my Raspbery Pi 3 (running Raspbian Buster) from my Windows machine. However, from inside the RPi, I am not able to ping/curl/wget/SSH any network resources:

• Remote domain names (ping google.com sends packages but receives none)
• Remote IPs (ping 1.1.1.1)
• LAN IPS (ping 192.168.0.120, my Windows box)
• The LAN router (ping 192.168.0.1)

Each of these operations time out.

Configuration

/etc/network/interfaces

source-directory /etc/network/interfaces.d

/etc/dhcpcd.conf

hostname 
clientid
persistent
option rapid_commit
option interface_mtu
require dhcp_server_identifier
slaac private
interface eth0
static ip_address=192.168.0.64/24
static routers=192.168.0.1
static domain_name_servers=127.0.0.1

Example ping to LAN router

root@rpi3:~# ping 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.
^C
--- 192.168.0.1 ping statistics ---
6 packets transmitted, 0 received, 100% packet loss, time 190ms

ifconfig -a eth0

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
inet 192.168.0.64  netmask 255.255.255.0  broadcast 192.168.0.255
inet6 fe80::ba27:ebff:fe9a:d30f  prefixlen 64  scopeid 0x20<link> 
ether b8:27:eb:9a:d3:0f  txqueuelen 1000  (Ethernet)
RX packets 1598  bytes 173011 (168.9 KiB)
RX errors 0  dropped 0  overruns 0  frame 0
TX packets 4178  bytes 434471 (424.2 KiB)
TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

iptables -L

root@rpi3:~# sudo iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

iptables --table nat --list --verbose

Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

routes -n

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.0.1     0.0.0.0         UG    202    0        0 eth0
192.168.0.0     0.0.0.0         255.255.255.0   U     202    0        0 eth0

Router information

MAC Address:           E4:6F:13:FA:CC:C3
IP Address:            172.103.149.21
Subnet Mask:           255.255.255.192
Default Gateway:       172.103.149.1
Primary DNS Server:    192.168.0.64
Secondary DNS Server:  1.1.1.1

Other information

• I am using this RPi to run PiHole. It was working happily before I re-arranged the office. PiHole is still running but does not function properly, because it cannot send network requests.
• I have the following crontab:

0 0 * * Sun apt update && apt-upgrade -y
0 0 * * Sun pihole -up

Things I have tried

• Power cycle router and RPi (obviously)
• Remove reserved IP for RPi on the router
• Various configuration changes to /etc/network/interfaces, as described here.

My question

How is it possible that I am able to SSH into the RPi and execute all the above commands, but the RPi isn't even able to access hosts on the LAN?

UPDATE with info from a comment:
I re-installed Raspberry Pi OS, and verified that everything was working again. After installing PiHole, the problem reproduced. It seems that PiHole has either changed some configuration or one of its dependencies is causing the issue.