I'm not aware of any difference.
Please attach a full log file showing just the login with initial directory listing using both 4.3.9 and 5.1.
To generate log file, enable logging
, log in to your server and do the operation and only the operation that causes the error. Submit the log with your post as an attachment. Note that passwords and passphrases not stored in the log. You may want to remove other data you consider sensitive though, such as host names, IP addresses, account names or file names (unless they are relevant to the problem). If you do not want to post the log publicly, you may email it to me. You will find my address (if you log in) in my forum profile
. Please include link back to this topic in your email. Also note in this topic that you have emailed the log.