What's the operating system of your PC? Did you enable WLAN AutoConfig(Win7&Vista) or Wireless Zero Configuration(WinXP). http://www.tp-link.com/en/article/?faqid=306...

Are you using google chrome? Try internet explorer.

Ross, W8980 supports NAT loopback indeed. Did you try http://88.247.135.100/ instead of http://88.247.135.100/activecollab/public/index.php in external network?

Firmware version is 141020? The client connected to repeater can ping the server successfully or not? How did you access it? How did you access interent via VPN connection? What kinds of VPN...

Keep testing and observing... If the problem comes back and you can still login the web interface, the system log should help to troubleshoot the problem.

If firmware upgrade won't help, try to change channels like 147 or others. Place the client connecting to 5GHz network next to the router and test if the connection will be stable. If the problem...