IPv6 has no relation to the DHCP server. All that checkbox does is either add rules blocking all IPv6 or not add them. Neither way has any impact on anything with IPv4.
Is the DHCP server running when it stops giving leases? Check Status>Services, and the system logs. The only time I've seen the DHCP server stop handing out leases is when it doesn't have any to hand out. It may be possible there is some missing input validation somewhere that lets you generate an invalid config file that stops dhcpd, which would be in the logs, but that would have to be something atypical you're doing.