We asked for additional information using e-mail on Tuesday, but received no reply so far.
The relevant reply to PASV command looked like this: "227 Entering Passive Mode (xxx)"
There are two possibilities. If this was in fact in the original log, it would explain the issue - "xxx" is definitely not an IP address, which makes this reply unparsable.
However, it seems more likely that the original log contained something else, which you replaced in order not to disclose sensitive data.
Unfortunately, doing this would have made it impossible for us to figure out what is going on.
Is this actually what the server sent? If not, would it be possible to send us a slightly more faithful representation of the original response?