It seems that your Time Capsule is defective, failing to access files. Apple has not offered recovery for Time Capsule, the last chance you can have a try is to remove the disk of Time Capsule out and connect it to a functional Mac device for file retrieval. This way is quite risky for two aspects, one is that you must ensure to take out the hard disk correctly to avoid it being physically damaged, and the second is that this step violates Time Capsule's warranty, cutting the chance of asking for a replacement of the problematic Time Capsule. So please think twice before you do it.
Moreover, backing up your data on cloud storage is recommended for which less likely gets broken or inaccessible.