libcurl would reuse a previously created FTP connection even when one or more options had been changed that could have made the effective user a very different one, thus leading to the doing the second transfer with wrong credentials.

libcurl keeps previously used connections in a connection pool for subsequent transfers to reuse if one of them matches the setup. However, several FTP settings were left out from the configuration match checks, making them match too easily. The settings in questions are CURLOPT_FTP_ACCOUNT, CURLOPT_FTP_ALTERNATIVE_TO_USER, CURLOPT_FTP_SSL_CCC and CURLOPT_USE_SSL level.