It's a common error happening when there's a difference in compression configuration between the server and the client.

comp-lzo disabled on server but enabled on client side.

After server add topology:

conf

server 10.1.2.0 255.255.255.0
topology subnet

OpenVPN server running in Proxmox KVM VM.

If you want to use KVM, you need to set your CPU type to at least to Intel Westmere or to host.

Westmere formerly Nehalem-C (Core i 1 gen)

“x86-64-v2-AES”:

• i5-3470S = IvyBridge (3rd gen)
• i7-3770K = IvyBridge (3rd gen)
• E5-2650 v2 = Ivy Bridge EP (3rd gen)

In OpenVPN server logs:

error:0A00018E:SSL routines::ca md too weak

workaround:

.conf

tls-cert-profile legacy
tls-cipher "DEFAULT:@SECLEVEL=0"

error=CA signature digest algorithm too weak:

Solution: upgrade server CA to use at least SHA256

Workaround:

client.conf

# to work around the cert too weak issue 
tls-cipher "DEFAULT:@SECLEVEL=0"

And from man 3 SSL_CTX_set_security_level:

Level 0

    Everything is permitted. This retains compatibility with previous versions of OpenSSL.

Level 1

    The security level corresponds to a minimum of 80 bits of security. Any parameters offering below 80 bits of security are excluded. As a result RSA, DSA and DH keys shorter than 1024 bits and ECC keys shorter than 160 bits are prohibited. All export cipher suites are prohibited since they all offer less than 80 bits of security. SSL version 2 is prohibited. Any cipher suite using MD5 for the MAC is also prohibited.

/sbin/resolvconf: 31: kill: Operation not permitted

Reason: openresolv: resolvconf fails if called from openvpn during system start with "kill: Operation not permitted"

Problematic script: /lib/resolvconf/libc.d/avahi-daemon

LZO compression is disabled on server but used on client.

Solution:

explicitly disable comp-lzo no on server.