If you encounter IPAM Access errors upon installation, check the following:

1. (un)tick the checkbox right to the Server Type field in your IPAM Server Inventory dashboard (sometimes it helps)
2. make sure that the firewall rules are right (you'll probably need a couple of additional rules added)
3. if DNS server errors occur, enable event log monitoring on a managed DNS server (this part is done via regedit - please continue reading here)
4. the last thing that helped me get rid of the errors was to add the IPAM Server AD account to the Administrators group (you could end with the account added to DHCP Users, DnsAdmins, Event Log Readers and Administrators groups, just as pictured below):