You can have a look into <path-to-repository>/keys/ and view the file inside (do not post it here, it contains sensible data). It should contain a valid JSON string. It may got corrupted on your system.
Easy fix, just add a new repository and remove the corrupt one.
Also Restic offers various commands to repair a corrupt repository, you can try that too. More information can be found at restic.net or their forums:
https://forum.restic.net/
Mit freundlichen Grüßen / Best regards
Alexander Mahr
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