The user performing the transfer of the case, either to themselves, or to another user, must also be able to access the account the case belongs to. This error has nothing to do with the recipient of the case's permissions on the account. The administrator can obviously view all records, so there's no problem when they transfer the case. However, they should be aware that they may be granting access to an account the user shouldn't be able to see otherwise (known as "implicit sharing"). This rule is in place so that a user can't "share" an account to a user accidentally.