Hello Plamen,
The convert process asks for a name for the new virtual machine (is not possible to use the same name of an existing VM into the VC), but it's possible that the VM had some manual renaming at any time.
The converter error was "The virtual machine "XXXXX" already exists", despite of that there isn't any virtual machine into the destination virtual center with that name. I can't export any logs because there isn't any converter task related to this (Is it possible that it hasn't created any job?)
Best regards,
Miguel