The error message clearly indicates OpenVPN cannot locate the certificate files during startup. The key log entries show:

Options error: --cert fails with 'client.crt': No such file or directory
Options error: --key fails with 'client.key': No such file or directory

While you mentioned the files exist in /etc/openvpn/easy-rsa/keys/, OpenVPN by default looks for them in the configuration directory (/etc/openvpn/). There are three ways to resolve this:

Option 1: Copy certificates to default location

cp /etc/openvpn/easy-rsa/keys/{server.crt,server.key,ca.crt,dh2048.pem} /etc/openvpn/

Option 2: Update server.conf with full paths

# Original:
ca ca.crt
cert server.crt
key server.key

# Modified:
ca /etc/openvpn/easy-rsa/keys/ca.crt
cert /etc/openvpn/easy-rsa/keys/server.crt
key /etc/openvpn/easy-rsa/keys/server.key

Even with correct paths, OpenVPN (running as nobody) needs proper permissions:

chmod 644 /etc/openvpn/easy-rsa/keys/{server.crt,ca.crt,dh2048.pem}
chmod 600 /etc/openvpn/easy-rsa/keys/server.key
chown nobody:nobody /etc/openvpn/easy-rsa/keys/{server.crt,server.key,ca.crt,dh2048.pem}

The sysctl errors indicate missing kernel modules. For OpenVZ containers:

# Load required modules (on host node)
vzctl set CTID --save --capability net_admin:on
vzctl set CTID --save --devnodes net/tun:rw

For better troubleshooting, run OpenVPN in debug mode:

openvpn --config /etc/openvpn/server.conf --verb 6

Check for these critical startup phases in output:

1. TLS certificate verification
2. TUN/TAP device initialization 
3. Network route configuration

OpenVZ requires special NAT configuration for tun interfaces:

iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o venet0 -j SNAT --to-source YOUR_MAIN_IP

Validate these essential components in your setup:

1. /etc/openvpn/server.conf paths
2. Certificate files exist in specified locations
3. IP forwarding enabled (sysctl -w net.ipv4.ip_forward=1)
4. Firewall rules for UDP 1194
5. SELinux context (if enabled): chcon -t openvpn_t /etc/openvpn/*

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The error message clearly indicates OpenVPN cannot locate the certificate files during startup:

Options error: --cert fails with 'client.crt': No such file or directory
Options error: --key fails with 'client.key': No such file or directory

First verify the actual location of your certificate files. Even though you mentioned they exist in /etc/openvpn/easy-rsa/keys/, OpenVPN might be looking elsewhere. Run:

ls -la /etc/openvpn/easy-rsa/keys/client.*
stat /etc/openvpn/easy-rsa/keys/client.crt

Your server.conf needs absolute paths to certificate files. Modify these lines:

ca /etc/openvpn/easy-rsa/keys/ca.crt
cert /etc/openvpn/easy-rsa/keys/server.crt
key /etc/openvpn/easy-rsa/keys/server.key

OpenVPN runs as nobody user, so certificates need proper permissions:

chown nobody:nobody /etc/openvpn/easy-rsa/keys/client.*
chmod 600 /etc/openvpn/easy-rsa/keys/client.*

The sysctl errors about bridge modules are unrelated but should be fixed:

modprobe bridge
lsmod | grep bridge

Run OpenVPN in debug mode for more details:

openvpn --config /etc/openvpn/server.conf --verb 6

For OpenVZ containers specifically:

vzctl set CTID --devnodes net/tun:rw --save
vzctl set CTID --capability net_admin:on --save

Create a verification script to check paths:

#!/bin/bash
for cert in ca.crt server.crt server.key client.crt client.key; do
    if [ ! -f "/etc/openvpn/easy-rsa/keys/$cert" ]; then
        echo "Missing: $cert"
    else
        echo "Found: $cert"
    fi
done

If issues persist, regenerate certificates:

cd /etc/openvpn/easy-rsa/
source ./vars
./clean-all
./build-ca
./build-key-server server
./build-key client1