RussellArdo thanks , your comments helped me to find solution
I also have two standart vSwitchs , one for all vmk inderfaces (10.10.10.0/24 for example) and second for VM network (10.10.11.0/24)
Host for migration and Converter were placed in 10.10.10.0/24 , but helper vm can get interface only in VM network - 10.10.11.0/24
These networks are not crossed in my environment and the converter was failed by timeout on the stage
Connecting to the Converter helper server on the destination virtual machine.
I temporary added VM network to vSwitch0 (10.10.10.0/24) and the converting process finished successfully.
The document is described how the process works