More and more connections fail

[--IP--]

As of late, more and more people are unable to connect to my server. The server itself runs FileCOPA and everyone using it have converted to the FlashFXP doctrine.

Everyone uses the latest version of FlashFXP. Some get an error with some strange <implicit> tag in the log, which i don't get myself. I am not sure it's just a FlashFXP bug so i won't create a ticket just yet. However, trying to connect with a different tool, WinSCP, allows them to connect right away. We all prefer FlashFXP obviously, so we would like to have that working.

I was hoping some experienced people here would have an idea what the issue may be.
The server runs SSH2 - SFTP v5 with public key authentication.
No settings were changed on the server nor client side since it stopped working for some. I have verified their settings myself via VNC, so all should be good.

Fail Log:

[11:35:00] [R] Connecting to Server -> DNS=files.server.com IP=0.0.0.0 PORT=22
[11:35:00] [R] Connected to  Server
[11:35:01] [R] Host key algorithm ssh-rsa, size 4096 bits.
[11:35:01] [R] Fingerprint (MD5): 74:ea:65:26:f0:93:e7:4b:12:e6:e5:69:4d:ef:26:7f
[11:35:01] [R] Key exchange: diffie-hellman-group-exchange-sha1. Session encryption: aes256-gcm, MAC: <implicit>, compression: none.
[11:35:02] [R] Connection failed

Success Log:

[R] Connecting to Server -> DNS=files.server.com IP=0.0.0.0 PORT=22
[R] Connected to Server
[R] Host key algorithm ssh-rsa, size 4096 bits.
[R] Fingerprint (MD5): 74:ea:65:26:f0:93:e7:4b:12:e6:e5:69:4d:ef:26:7f
[R] Key exchange: diffie-hellman-group-exchange-sha1. Session encryption: aes256-cbc, MAC: hmac-sha1, compression: none.
[R] Auth Type: Public Key
[R] Authentication succeeded
[R] SSH Connection open
[R] Connection established with FileCOPA (SFTP v5)
[R] SFTP Connection Ready
[R] Retrieving directory listing...
[R] List Complete: 876 bytes in 0,19 seconds (0,9 KB/s)

What differs seem to be the MAC: <implicit> / hmac-sha1. I'm not an expert on this yet, so I'm hoping for some bright minds I just find it odd when the settings are identical to mine, but the result is different.