My airport extreme won't accept my password on my Mac

Just typical Yosemite..

Reset the Airport Extreme to factory and redo the setup from the computer.

Here is the full list of recommends but the main one is just the factory reset.

Factory reset universal

Power off the AE.. ie pull the power cord or power off at the wall.. wait 10sec.. hold in the reset button.. be gentle.. power on again still holding in reset.. and keep holding it in for another 10sec. You may need some help as it is hard to both hold in reset and apply power. It will show success by rapidly blinking the front led. Release the reset.. and wait a couple of min for the AE to reset and come back with factory settings. If the front LED doesn’t blink rapidly you missed it and simply try again. The reset is fairly fragile in these.. press it so you feel it just click and no more.. I have seen people bend the lever or even break it. I use a toothpick as tool.

Then redo the setup from the computer with Yosemite.

1. Use very short names.. NOT APPLE RECOMMENDED names. No spaces and pure alphanumerics.

eg AEgen5 and AEwifi for basestation and wireless respectively.

Even better if the issue is more wireless use AE24ghz and AE5ghz with fixed channels as this also seems to help stop the nonsense.

2. Use all passwords that also comply but can be a bit longer. ie 8-20 characters mixed case and numbers.. no non-alphanumerics.

3. Ensure the AE always takes the same IP address.. this is not a problem for you as the cable modem SB5102 has no router.

4. Check your share name on the computer is not changing.. make sure it also complies with the above.. short no spaces and pure alphanumeric..

5. Make sure IPv6 is set to link-local only in the computer. For example wireless open the network preferences, wireless and advanced / TCP/IP.. and fix the IPv6. to link-local only.

There is a lot more jiggery pokery you can try but the above is a good start.. if you find it still unreliable.. don't be surprised.

Tell me if you still have issues.

The AirPort Extreme has two passwords.....one is the Base Station or Device password.....and the other is the Wireless Network password. If you have added a hard drive to the AirPort Extreme, then the drive may have a disk password as well.

If you are opening AirPort Utility on your Mac, the application normally is looking for the Base Station or Device password....not the wireless password, unless you specified during the original set up that you wanted to use the same password for both the Base Station and Wireless.

If you use the iPhone or iPad that you used to originally set up the AirPort Extreme, then it has stored the passwords that you need and enters them automatically. That's why you are not asked to enter a password when you use the iPhone or iPad.

If you have AirPort Utility installed on the iPhone or iPad, you can "see" or "retrieve" your passwords as follows:

Open AirPort Utility on the iOS device

Tap on the picture of the AirPort Extreme

Tap Edit at the upper right corner of the screen

Tap Advanced

Scroll down and tap on Show Passwords, and they will be revealed

Otherwise, you can perform a Soft Reset on the AirPort Extreme to reset all the passwords. It might be a good idea to write those down. For more info, see this Apple support document and look for the info on a Soft Reset.

Resetting an AirPort base station FAQ - Apple Support

If all else fails, then you will need to perform a Factory Default Reset on the AirPort Extreme and then set it up again.