OpenVPN: TLS error

I am trying to connect a laptop running Linux Mint to a FreeBSD server running OpenVPN with no luck. I keep seeing TLS errors like this in the server log:

Tue Sep 17 23:14:51 2013 us=127496 Authenticate/Decrypt packet error: packet HMAC authentication failed
Tue Sep 17 23:14:51 2013 us=127570 TLS Error: incoming packet authentication failed from [AF_INET] \\my IP number\\

I have tried using TCP instead of UDP, authenticating by password, and various other options, all to no avail. Here is the server config:

daemon
port 1194
proto udp
dev tun
ca      /usr/local/etc/openvpn/keys/ca.crt
cert    /usr/local/etc/openvpn/keys/server.crt
key     /usr/local/etc/openvpn/keys/server.key
dh      /usr/local/etc/openvpn/keys/dh2048.pem
server 192.168.100.0 255.255.255.0
tls-server
tls-auth /usr/local/etc/openvpn/keys/ta.key 0 # This file is secret
keepalive 10 120
duplicate-cn
user nobody
group nobody
persist-key
persist-tun
status                  /var/openvpn-status.log
log-append              /var/log/openvpn.log
verb 5

I generated the ca.crt and ca.key files on the server and copied them to the client along with ta.key, after which I created the necessary client certificates.

I am using Gnome Network Manager to configure the OpenVPN client. Not sure where it is writing its settings.

Any thoughts about what might be going wrong here?

TIA

As per @DerfK comment. You need to apply correct key-direction.

Edit your VPN connection in your Network Manager.

Go to VPN tab.

Click Advanced.

Go to TLS Authentication tab

Change the Key Direction (at the bottom) to 1

Have you transferred the TA.key file to the client? This file is a shared secret. It must be available on the client, and referenced by the configuration.

That feature isn't required though, you could disable it on the server.