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[Openvpn-users] troubleshooting "HMAC authentication failed"

I have several old-school OpenVPN tunnels between a rack-mounted server
and several laptops and an OpenWRT.

I couple of days ago I couldn't use my laptop to ping machines behind
the OpenWRT.  Eventually I figure out that the OpenWRT can no longer
properly communicate with the hub server.

When I run it with --verb 6 I see

Authenticate/Decrypt packet error: packet HMAC authentication failed

from the machine receiving pings.

I have checked the time (and synchronized it with ntpclient), I have run
checksums on the key files.  I am unable to find anything that changed
on the OpenWRT.  Laptop openvpn tunnels to the hub server work (that's
how I can log on to the router from outside, I tunnel to a laptop and
then ssh to the OpenWRT).  Laptop tunnels to the OpenWRT (since wireless
security is rather weak) do not work, so I'm focusing my blame on it
right now.
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