It seems that SE fails to connect to SFTP servers that only support newer sha2 algorithms.

The server has the following key exchange algorithms enabled:
ecdh-sha2 over secp256k1
diffie-hellman-gex-sha256

The server has the following data integrity protection algorithms enabled:
hmac-sha2-256

SE won't connect to this server unless older sha1 algorithms are added.

I think maybe it's as simple as updating the Ganymed library that SE uses. Thanks and please let me know if you have any questions.