Extending Time Capsule to Airport Express using Ethernet.

This sounds as if you have originally configured the AirPort Express to "extend a wireless network" so that it is set up to connect to the Time Capsule using a wireless connection.

If this is the case, and you then try to connect the AirPort Express to the Time Capsule using an Ethernet cable, the Express now sees two different connections.......wireless and wired.....and it does not know what to do. So, it crashes and not much of anything works in regard to the AirPort Express.

My suggestion would be to start over again and perform a Hard Reset on the AirPort Express to erase the current network settings and then use the AirPort Utility setup "wizard" to automatically configure the AirPort Express correctly to extend using an Ethernet cable connection.

If this sounds like something that would like to try......

Power up the AirPort Express for a few minutes.....if is not already powered on.....and then hold in the reset button on the Express for 7-8 seconds. Then, release the reset button and allow a full minute for the AirPort Express to restart to a slow, blinking amber light status.

Connect the Ethernet cable......(we assume that you have tested the cable to make sure that it is working correctly).......from one of the three LAN <--> ports on the Time Capsule to the WAN "O" port on the AirPort Express

Click the WiFi menu at the top of your Mac's screen

Look for a listing of New AirPort Base Station

Just under that listing, click directly on AirPort Express

When you click on AirPort Express, that will start up the AirPort Utility setup "wizard". The wizard will take a few seconds to analyze the network and then display a screen that looks like the example below except that you will see your devices pictured

Type in a short simple device name for the AirPort Express

Click Next

Watch the next screen that appears very carefully to make sure that the AirPort Express is being set up to "extend using Ethernet". This will confirm that the wizard has picked up the Ethernet cable connection between the Time Capsule and AirPort Express so the correct settings will be applied to the AirPort Express

When you see the message of Setup Complete, click Done.......and you are done.

If you are curious, you can now use AirPort Utility to take a look at the Wireless settings that were applied automatically to the AirPort Express to work with an Ethernet connection

You may be surprised when you see how the AirPort Express has actually been configured with the correct settings.

Preparing

Reading Previous Settings.

I have no problems when I set up a spare AirPort Express as a test to extend using Ethernet, so I am not able to duplicate the issues that you are are reporting.

Something is wrong. If the Hard Reset on the AirPort Express worked correctly, there are no "previous settings" to read, since they should have been erased. Possible defective AirPort Express. Not sure how much time that you want to spend on this, but there are more things to try if you have the time and inclination.

Try a factory default reset......which is more difficult to perform, since you almost need 3 hands to do this.

Power off the Express

Hold in the reset button first, and keep holding it in for an additional 7-8 seconds while you simultaneously plug the power back into the Express

Release the reset button after the 7-8 second hold, and give the Express a minute to restart

An Unexpected Error Occurred

This is another separate issue that is usually caused by incorrect IPv6 settings on your Mac computer. There are two things that you can try to possibly eliminate the setup errors:

1) Try using an iPhone or iPad for the setup of the AirPort Express. If you want to try this......

Reset the Express back to default settings again

Go to the Home Screen on the iPhone or iPad

Tap Settings

Tap WiFi

Tap on AirPort Express under the listing of Set Up New AirPort Base Station

The setup is basically the same as the example already posted above

2) Check/change the IPv6 settings on your Mac and then restart the Mac again as well as starting over again on the Express

If you want to try this......

Open System Preferences (the gear icon on the dock)

Open Network

Click on WiFi on the left panel to highlight it

Click Advanced at the lower right of the window on the right

Click the TCP/IP tab at the top of the next window

Check the setting for Configure IPv6.......it should read Link Local Only

If it does not, change the setting to read Link Local Only

Click OK

Click Apply near the bottom of the next window

Restart the Mac again

Try the setup again

It would not hurt to also power off the Time Capsule, wait a few seconds, then power it back up again before you try to set up the AirPort Express.

Nice work! Looks like the IPv6 settings were the missing ingredient here.

If you are curious and want to take a look at the actual settings that were applied to the AirPort Express using AirPort Utility, you may be surprised to see that it was not really set up to "extend" at all.