Maybe I stressed it too much...we have 2 class B addresses that we manage. I got the error but when I went back to the IPAM page it looks like all 3/4's of the IP ranges imported and started the scanning. Looks like it stops at 10.14.179.X subnet. When I ran our other class B I got the same website error and fewer results. Looks like it stops at 10.15.84.x subnet.
Good catch, this is filed, as a temp workaround you can go back into the Subnet Allocation Wizard and walk through this process again and ensure the checkbox is selected to filter out existing subnets and then create.
Let me know if this works for you.