Time Capsule defect due to faulty hard drive
Apple has used Seagate Granada hard drives in the Time Capsules. These turn out to have a design flaw that causes them to break faster than normal. German media speak of a ticking time bomb and an imminent 'data death'. That seems a bit exaggerated, but it's good to have an extra backup and not rely entirely on the Time Capsule. Until 2018, Apple sold the Time Capsule, the backup system with which you can easily make a Time Machine backup. It uses a modified version of the Grenada hard disk (ST3000DM001 and ST2000DM001 2014-2018) that does not function properly.
This is the part of the hard disk where the read and write heads are parked when they are not active. These hard disks consist of two different materials that break over time due to temperature influences. Another factor is that the Time Capsule cannot be adequately ventilated. The hard disk read and write units lose their shape and damage the data disks. Often the hard disk can no longer be completely rescued and you can lose data. It was discovered by the German data recovery firm 030 Datenrettung Berlin, which has already seen several cases in practice. When a broken Time Capsule is handed in to this company, it is almost always about this problem. Sometimes the data can be retrieved with a lot of effort, but you better get ahead of the problem.
It is a fifth generation Time Capsule model number ME177Z/A (with Seagate Grenada ST2000Dm001) and ME182Z/A (with Seagate Grenada ST3000DM001 ). Ordinary computers equipped with such a hard disk can also run into problems. It would have been known since 2016 or longer, but Seagate has never staged a recall or anything like that. Neither does Apple.
If you're handy, you could replace the hard disk with a more recent model. You can find the manual for this at iFixit. For others, the best advice is to make a full backup of your current Time Capsule and look for a new hard drive for your backups.