ACK packet of TCP Three-Way-Handshake get lost

[JasMan]

Hey,
I've a stange issue with my clock radio, which is able to play Internet radio streams as wake-up sound.

Once or twice a week the radio is not able to start the stream when it should wake me up.
I did an capture on the LAN side of the OPNsense aplliance (see attachment). It looks like the clients ACK for the SYN-ACK never reached his destination. The server retransmit the SYN-ACK again and again, and the client retrasnmit his ACK again and again, too.
The firewall and IPS log showed no blocked packets. I will now capture the WAN side to be sure that the packet has left the OPNsense appliance.

Funny fact: this happens only when the stream is started by the wake-up timer. When I start the radio manually, the stream starts always without any problem.

Any idea what can cause this issue? Is there another log were I should have a look to be sure, that nothing was blocked?
Thanks.

Jas