When attempting to copy an encrypted RDS snapshot between AWS accounts, you may get the following error message:
Encrypted source snapshot: <snapshot ARN> cannot be copied because it does not exist in your account
This can occur if you are attempting to copy an encrypted RDS snapshot to a different AWS account AND a different AWS region.
If you are copying an encrypted RDS snapshot to another AWS account, the copy must be performed to the same AWS region.
Solution
To resolve this issue, copy the encrypted RDS snapshot to the other AWS account, but in the same AWS region.