I was having the same problem with some of my laptops (mostly older ones), IP address would be assigned by the router, the laptop would receive the address but then loose it. The reason code in the router log was 8. Looked the code up at the website previously mentioned a got a message of “Operating System moved the client to another access point using non-aggressive load balancing”.
So based on that and some other info I dug up in the logs I changed the WPA Mode on the router from “Auto” to “WPA2 Only” and presto no more problems. It actually didn’t matter if I choose “WPA Only” or “WPA2 Only” in both cases it resolved the problems I was seeing.
Although it did not appear to have a impact on the problem, at least on the 2 laptops I initially tested the fix with, I did subsequently changed the “Cipher Key” from “TKIP and AES” to just “AES” just to be sure.
Hope this helps.