problem with the FTPS connection, Error code 421

[ad20201216]

I am having a problem with the FTPS connection. Details are as follows:
server OS:Windows Server 2012 R2
server :Filezilla 0.9.43 beta
client OS:Windows 7
I use apache commons net 3.7.2 as base lib,and extend FTPSclient,my code as follow:

and i get client error

filezilla server's error log:

error id is 004056
This error occur randomly, and may occur after a few connections or dozens of connections.

[boco]

You are using an unsupported version of FileZilla Server. Please update to 0.9.60.2 as it's the only one supported.

[ad20201216]

thanks for u reply,but I can't upgrade the version of filezilla in production environment.

[botg]

Rant: I'm seriously considering to add a killswitch to all my products so that they _HAVE_ to be updated regularly...

[boco]

thanks for u reply,but I can't upgrade the version of filezilla in production environment.

Your "production environment" is in grave danger. That ancient version contains multiple severe security vulnerabilities actively being exploited in the wild. Additionally, no one here really knows anymore how that version worked and behaved, as we moved on.
For your own good, you really need to come up with a concept for doing proper server maintenance. Most simply, run a backup server to keep production up while upgrading. Or, prepare a fully updated FTP server with identical configuration, test it on a spare machine, then, do an in-place switch. Easy to do with FileZilla Server and keeps downtime short.

Rant: I'm seriously considering to add a killswitch to all my products so that they _HAVE_ to be updated regularly...

Good idea. You can't keep users from running old versions, as it's Open Source Software, after all. But those forks will not get any support here, as the GPL license states.