It took me a few times before I could access the database too. I think it is just unreliable. I gave my amp a static IP and that seemed to fix it. (I think the amp has a hard time finding DNS servers) Pick a unused IP address outside your DHCP pool with the last number between 1 and 253. My network uses addresses like 192.168.123.XXX yours may be different. Just substitute yours addresses for mine.
Turn off DHCP and enter something like this...
A:192168123_99 (IP address)
S:255255255__0 (Subnet Mask)
G:192168123254 (Gateway AKA. your router)
1:XXXXXXXXXXXX (DNS server #1 - get this info from your router unless you use the hack above)
2:XXXXXXXXXXXX (DNS server #2 - get this info from your router)
You don't need to use port forwarding because your Amp is not a server. Requests can still leave your network and your router knows and waits for a response. If the router is not expecting a request, it ignores it. That is where port forwarding comes in, forcing the router to listen to requests it wasn't expecting.The PC software...
Port forwarding won't do anything since the Amp and your computer are both on your LAN. If it is pointing to your PC, you just let users on the internet access to one of your computers. (Specifically your music using that Onkyo software) While I don't think any bad programs would take advantage of this open port, there is still a security issue to think about.
Try pinging your Amp in the command line from your PC. Type "ping 192.168.123.100" (you change the IP address to point to your amp.) If it does communicate, everything is configured correctly on your amp and your computer is the problem. Check for a firewall on your PC. It could be blocking communication.
If Ping fails, double check your settings on the Amp. Most likely they are pooched.