Client on Azure VM - issues

[Lats]

We have an on-premise FTP server (Using Win 2008 R2 and IIS) which we have been using with other on-premise web servers for transferring files.

As we are in the process of migrating these web servers to Azure, we encountered an issue where the FTP server connections are getting disconnected.

We used multiple client tools from Azure web server such as FileZilla and we can see the connection is being established (using port 21). This can happen as we click on the folder structure to browse or start transferring files, we get the odd disconnection and error messages:

2020-03-23 17:06:35 17320 2 Response: 501 Server cannot accept argument.
2020-03-23 17:06:35 17320 2 Command: PASV
2020-03-23 17:06:35 17320 2 Response: 227 Entering Passive Mode (71,41,31,41,116,222).
2020-03-23 17:06:35 17320 2 Command: RETR gpm_APDLOGIC_GPMF00N4_4pm.20180719T0500-04.csv
2020-03-23 17:06:35 17320 2 Response: 150 Opening BINARY mode data connection.
2020-03-23 17:06:35 17320 2 Response: 550 The network connection was aborted by the local system.
2020-03-23 17:06:35 17320 2 Error: File transfer failed
2020-03-23 17:06:35 17320 2 Status: Starting download of /IDCT/gpm_APDLOGIC_GPMF00N4_4pm.20180719T0500-04.csv
2020-03-23 17:06:35 17320 2 Command: PORT 172,21,208,29,26,88
2020-03-23 17:06:35 17320 2 Response: 501 Server cannot accept argument.
2020-03-23 17:06:35 17320 2 Command: PASV
2020-03-23 17:06:35 17320 2 Response: 227 Entering Passive Mode (71,41,31,41,116,216).
2020-03-23 17:06:35 17320 2 Command: RETR gpm_APDLOGIC_GPMF00N4_4pm.20180719T0500-04.csv
2020-03-23 17:06:35 17320 2 Response: 150 Opening BINARY mode data connection.
2020-03-23 17:06:35 17320 2 Response: 550 The network connection was aborted by the local system.
2020-03-23 17:06:35 17320 2 Error: File transfer failed
2020-03-23 17:06:35 17320 2 Status: Starting download of /IDCT/gpm_APDLOGIC_GPMF00N4_4pm.20180719T0500-04.csv
Networking team have assured me that they are not seeing anything being dropped in the firewalls.

Anyone have any suggestions?

[botg]

Most likely a firewall or NAT router interfering with the connection. The error message sent by the server suggests it's located server-side.