MBA not backing up to Time Capsule

[thizisweird]

Long story made short: a friend was liquidating assets of a business, and I bought his Time Capsule. I set up my dad's Pro tower to use Time Machine (which is wired into the DSL modem) and it works just fine. My 2015 MBA, on the other hand, refuses to back up. I keep getting Error 45: "The backup disk image... could not be accessed." I've tried updating the password, reselecting the Time Capsule as a backup volume, resetting the Time Capsule, all to no avail.

I'm already considering reformatting the MBA (it was from my buddy's liquidation as well), but I'd rather just wait for my SSD upgrade to bother with all that. I don't want to have to go through the whole process twice, and I'm not going to mirror the drive (not my thing, and it'll take a long time).

Any ideas? I'm wondering if this problem will go away when I do a fresh install of Mojave... but without much input from the more experienced, I'm at a loss. Luckily there's nothing on the MBA I need backed up, and the SSD is too small to do serious work on it... but I'd still like to get this issue solved before I even order the new SSD.

 

[Mitthrawnuruodo]

Have you tried verifying the backups:

Check your backup
If you back up to a server, AirPort Time Capsule, or drive connected to your AirPort Extreme Base Station, make sure that your current Time Machine backup doesn't have any issues that could prevent further backups:

1. While holding down the Option key on your keyboard, click the Time Machine menu
   
   in the menu bar, then choose Verify Backups.
2. If Time Machine finds an issue with your backup, it displays a message with details. Follow the onscreen instructions.

This and more on the page: If you can't back up or restore your Mac using Time Machine