Error unprotecting the session cookie.The key {...} was not found in the key ring.

 .Net Core 3.0

续：Error unprotecting the session cookie.The payload was invalid. 出现了新的BUG

warn: Microsoft.AspNetCore.Session.SessionMiddleware[7]
      Error unprotecting the session cookie.
System.Security.Cryptography.CryptographicException: The key {40d34e51-8e22-41b3-a179-fe353bcd0280} was not found in the key ring.
   at Microsoft.AspNetCore.DataProtection.KeyManagement.KeyRingBasedDataProtector.UnprotectCore(Byte[] protectedData, Boolean allowOperationsOnRevokedKeys, UnprotectStatus& status)
   at Microsoft.AspNetCore.DataProtection.KeyManagement.KeyRingBasedDataProtector.DangerousUnprotect(Byte[] protectedData, Boolean ignoreRevocationErrors, Boolean& requiresMigration, Boolean& wasRevoked)
   at Microsoft.AspNetCore.DataProtection.KeyManagement.KeyRingBasedDataProtector.Unprotect(Byte[] protectedData)
   at Microsoft.AspNetCore.Session.CookieProtection.Unprotect(IDataProtector protector, String protectedText, ILogger logger)

把浏览器关闭再打开，就可以了。