I tried to add to CPDB two participants from an existing survey, and immediately after was not able to access the CPDB because "Argument 1 must be at least CRYPTO_SIGN_BYTES long".

Having direct access to the MySQL database, I identified and looked into the table named lime_participants, and saw that all other participants had their own "firstname" and "lastname" encrypted, whereas these fields were plainly readable (unencrypted) for the two participants transferred from my survey.

I had them manually removed (via the MySQL commands:

delete from lime_participants where firstname="Aaron";
delete from lime_participants where firstname="Zack";

) and the system started working again (i.e. I could again access the CPDB from the website).

I conclude that the script which transfers participants from one survey to the CPDB does not pre-encrypt the required fields.