ARP Ping is working but IP Ping not, why?

RouterOS Beginner Basics
1

In my network I’m able to ping a device using the ARP, but when using the IP Ping methode I cannot reach the Mikrotik AP. Any idea’s? What could be the reason?

The Mikrotik AP is connected to a HP switch which has another Mikrotik AP, ping that 2nd AP using IP and/or ARP is working fine. So I can reach the switch to which the AP’s are connected.
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2

Pinging “the problem AP” from the 2nd AP on the same HP switch, is working.
Pinging the device from which I cannot do the ping from the AP is working. In other words, the reserve IP ping is working.

3

Firewall?

4

Nope, then I would not be able to ping the other AP on the same switch.
Firewall is not enabled in this section of the network.