Hi Thomas linux4guru,
This has happened to me in the past, in my scenario it was caused by the connection being dropped numerous times during the process and Serv-U thought it was some sort of attack which resulted in a blocked IP.
Unfortunately, I never got down to resolving the issue as we were changing the way our network worked, it resolved itself after all the changes we were making to our environment... However, I hope it may help you try to locate the issue.
I figured it out why this happened to me.
Global --> Limits & Settings --> Settings tab --> Uncheck the box "Block users who connect more than "x" times within "x" seconds for "x" minutes (0 for permanently).
This was my fix. I no longer have issue after removing this.