This particular system was never 1.2.3. The original install was a 2.0 snapshot and it's been through a few updates since. In my case the dhcp address range seems to have stayed in the config file when I deleted an OPT interface. when I created a new OPT interface later, the legacy dhcp information for that interface stuck to it, and the address range was at that point invalid.
You may want to check the dhcpd section of your config file for errors that might be causing an issue. You can download just this section ("dhcp server") in the Diagnostics: Backup/Restore section. Mine looks something like this and is working fine at the moment (interfaces omitted for brevity):
<dhcpd><lan><range><from>192.168.85.51</from>
<to>192.168.85.60</to></range>
<defaultleasetime><maxleasetime><netmask><failover_peerip><gateway><domain><domainsearchlist><ddnsdomain><tftp><ldap><next-server><filename><rootpath><numberoptions><staticmap><mac>00:1e:8c:83:9b:93</mac>
<ipaddr>192.168.85.2</ipaddr>
<hostname>ren</hostname></staticmap></numberoptions></rootpath></filename></next-server></ldap></tftp></ddnsdomain></domainsearchlist></domain></gateway></failover_peerip></netmask></maxleasetime></defaultleasetime></lan></dhcpd>
If you find errors in yours, correct them, save the corrected file, then upload it again. You may have to restart your dhcp server at that point, or just reboot to test.