That's good, but, not necessarily this eliminates firewall problem. We can see a lot of problem with firewall and FTP. Even though the firewall is permitting the traffic, some FTP behavior can cause Firewall to drop connection. One good point is inspection, as you said you already have it, we need to keep investigating. But, yes, the FTP server is one possibility for sure.
First, try to identify if you are using Active or Passive FTP mode:
Active mode:
Client opens up command channel from client port 2000(a) to server port 21(b).
Client sends PORT 2001(a) to server and server acknowledges on command channel.
Server opens up data channel from server port 20(b) to client port 2001(a).
Client acknowledges on data channel.
Passive mode:
Client opens up command channel from client port 2000(a) to server port 21(b).
Client sends PASV to server on command channel.
Server sends back (on command channel) PORT 1234(a) after starting to listen on that port.
Client opens up data channel from client 2001(a) to server port 1234(a).
Server acknowledges on data channel.
As you can see there're different behavior in terms of port in different FTP mode and this can cause problem for Firewall.
Please, share the ASA config again but this time attach as txt file.
-If I helped you somehow, please, rate it as useful.-
