The version of phpseclib that comes with Magento 1 is super old. If you have issues with SFTP exports you can try updating phpseclib manually.
1) Log in via FTP
2) Go into root Magento directory
3) Go into the "lib" folder
4) Create a backup of the "phpseclib" folder. Then rename the folder to "phpseclib2".
5) Download and extract the following ZIP file and put the contents into "lib" so a new "phpseclib" folder is created. This is the updated version of phpseclib then.
6) Test everything and make sure the export as well as the store in general works.