Incompatible if more than a single connection is used, dependent on FTP server

[OPTIAIR]

V2.3 1993 seems to crash if more than a single connection is use against some servers.
It hangs on this screen for 5 download if I use 2 connections per host.
Xfering
Connecting
Pending
Pending
Pending

Single Connection, = OK
One per Host = OK
Multiple for each transfer = OK
the rest fails...
Maybe the Serv-U FTP server does not allow more the one connection... , it works with CrushFTP

[ForumAdmin]

Some servers will not allow multiple connections (or can't handle it in some cases).

If you post the full log there might be more information that helps diagnose the issue.

Better to always start with one connection, then work up from there.

[OPTIAIR]

The cause of the problem was using a ppk file and use Putty Compatible STFP checked in COMBINATION with "Keep Alive" hard checked. When disabling the Keep alive, "multiple for each transfer" works again. So I might have labelled it incorrectly "dependent on the FTP server".

dump:
coreftp build #1995 (Pro), caused an Access Violation (0xC0000005) in module ssh.dll at 0033:000000008F899AF2.

But that crash (ssh.dll) due to x parallel tracks => does that mean that the PuTTY 0.76 engine in CoreFTP does not handle multi-threading properly?
Are the x parallel connections waiting on each other (thread contention), keeping the combined speed stuck the same as the level of 1 single connection?
Can you reproduce by activating the "Use Putty Compatible SFTP" and test speed in single vs multiple for each transfer. Maybe test with a higher version of Putty.