OpenWrt ssh host identification with dyndns

If i ssh to my remote OpenWrt router ["via DynDNS"] 1. I get the message @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ 2. Ok, i delete the old line in ~/.ssh/known_hosts, add the new one 3. But my password wasn't accepted (even though I copy+pasted the good password) 4. But: if someone reboots the remote router it says: @ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @ 5. Ok, again, I delete the line in ~/.ssh/known_hosts, add the new one [the old one??], and presto! I can log in! Why? Is this because a dyndns update failed, and I'm trying to log in to the wrong IP? And if the router is rebooted, the dyndns IP will be updated, so that the IP is correct, and I can log in? Is that why it says “host identification failed”?
It happened for the 3rd time... I don't know what's exactly is going on.