Fix for me (using a C2300):

Log into router, click on "Advanced", choose the "System Tools" section, and "Time Settings" under that. Set both NTP servers to "pool.ntp.org" and reboot the router. Any existing OpenVPN configuration should now work again.

Explanation:

I was having a similar problem on the C2300 -- an exceptionally good router except for this bug. After some hard research and experimentation, I figured out my specific issue: the router's time.

Like most encrypted communication protocols on the internet, OpenVPN's protocols require the two communicating parties (VPN client and VPN server) have time that's roughly calibrated. The router calibrates its own time via NTP through nist.gov's servers. For some reason, the C2300's firmware dislikes these NTP servers, and so for me its time was perpetually stuck in the past leading to the VPN issues.

Luckily, pool.ntp.org works -- I set both time servers to this, problem solved. It pulls the right time after booting up, and OpenVPN works fine after reboot now, even with the latest firmware.