"backup failed." What next?

The password is not the culprit if all of your devices can connect to the network and Time Capsule "Data" drive appears as the target for Time Machine backups.

Try powering off the Time Capsule and your Mac and restarting both devices. If the same issue occurs, then unfortunately the message that you are receiving almost always indicates that the backups have become corrupted and are no longer useable.

Normally.....but not always.....a message like the one that you see below will appear when this occurs.

When the backups have become corrupted, all that you can do is manually erase the Time Capsule "Data" disk using AirPort Utility and start over again with a new backup and hope that the Time Capsule drive has not been physically damaged.

How do I erase the "Data" drive manually?

Open AirPort Utility on your Mac, select the Time Capsule, click Edit and select the Disks tab at the top of the next window

Click Erase Disk

Use the "Quick Erase" option, which will only take a few minutes

Then, try to run a new backup which will take some time, since everything on the Mac must be backed up

Can I access it somehow and run Disk First Aid?

No, remember that we said that Disk Utility / First Aid can only be run on a "local" drive.....not on a "network" drive like the Time Capsule.

The Time Capsule drive is a "network" drive that connects to the network.......not a "local" drive that connects directly to a Mac.....like a USB drive. Disk Utility is used on "local" drives.....not on "network" drives.

Hold down the option key on your Mac while you click on the Time Machine "clock" icon at the top of the Mac's screen.

Click on Verify Backups

The process will take anywhere from a few minutes to an hour or more depending on much data is stored on the Time Capsule drive

Post back on the results of the Verify Backups process