Just an FYI for anybody searching around for a solution to the "404 error" problem and comes across this thread but is still confused, here's what I did to fix this problem!
For reference, here's what I was troubleshooting and where I'm at now:
Cisco IOS Software, C2700 Software (AP3G2-K9W7-M), Version 15.3(3)JJ1, RELEASE SOFTWARE (fc3)
I tried using different iOS versions as previously mentioned but this never fixed the actual issue where EVERYTHING i did resulted in a 404 error. I could reach the web portal no problem, but everything after that point was the issue...Quick setup? Nope, 404 error. Update some setting? Nope, 404 error, etc.
I tried, ap3g2-k9w7-tar.153-3.JI5.tar, JI6.tar, JPJ3a.tar, and JJ1.tar. JPJ3a.tar seemed to make the problem worse...things didn't seem right and MORE information was missing...but that could've been just me. I ended up settling with JJ1.tar as my final firmware (currently using)
So, what I think ultimately fixed the issue was this - https://www.cisco.com/c/en/us/support/docs/security-vpn/public-key-infrastructure-pki/215118-ios-self-signed-certificate-expiration-o.html#anc2
Reading through this was a HUGE help! (This was already posted earlier in the thread by another user, so not trying to steal thunder :P just...it was SO helpful!) Specifically section "Workaround 2 - Use the IOS CA Server to generate a new certificate"
I wanted to cover all my bases, so i zero'd my RSA keys and started with scratch.
I had to configure SNTP on the AP (used 0.us.pool.ntp.org) and synced up date/time then went through and followed the instructions in Workaround 2. I then generated my RSA general-keys and then configured SSH 2.0 and http secure-server...after doing all that, I was FINALLY able to log into the web portal and use the Easy Setup without running into the 404 error after every change!
I hope this helps somebody, because I've been banging my head against the wall trying to figure this out before stumbling across this thread! :D
