I am not sure your analysis of the error is correct, there seems to be many reasons for that error code from the Credential Manager.

Regardless, when there is a size limit, the solution would be to generate a random encryption key, perhaps an AES-128 key, and encrypt the long data with it. Store the encrypted long data somewhere without that size restriction, typically the file system. Then store the encryption key instead in an appropriate way in the keyring.

To retrieve, get the key from the keyring and decrypt the long data as needed.