VLAN 0 (no such VLAN ID) should indicate "untagged" and typically generates that type of DHCP warning in Dashboard. So just out of curiosity, try this, click into that AP's page, click on the pencil next to LAN IP, and for the VLAN box, leave it blank, don't even put a "1" in there, just leave it blank and click save. Wait a few minutes and refresh the page, see if that does it. Should not have to reboot the AP, but try that too if it's not automatic.
To be sure, I just reproduced this exact issue in my home lab in just a few minutes by placing a VLAN 1 tag on an AP, got the warning, then blanked out the VLAN field, and warning goes away.