Hello,
I’ve got strange problem with WinBox… On only one router…
Leases tab on DHCP server is broken - how to repair it?
I’ve removed all files from appdata\roaming\miktoik\winbox already but with no luck 
RouterOS version 6.4 mipsbe.
Best regards,
AF
take backup using export command, then reset router OS as factory default then import configuration from previously exported file. If problem not solved then remove DHCP package from router, follow bellow steps.
login in to winbox
click on System ==> Package
==>Select DHCP package
==>Click on Uninstall
==> Reboot your router to uninstall DHCP package
then drag and drop same version DHCP package in to file menu of router
restart the router
Import your configuration.
and now your problem should solved already.
good luck
Hello,
/system reset-config… helped 
Thanks!