I had the same problem but when i was trying to upload the files using FTPS with explicit TLS 1.2 over an AWS Lambda function.
Each time that i was trying upload a file, there was an lambda timeout on the storbinary called, and the function ended whit error on each execution.
Finally the only solution that i founded, to solve this issue, was:

1- It using a threading to do the storebinary process.
2- Put an sleep depending do File size.
3- After the sleep function, it using ftplib.dir.