@hspindel 

Did you set a static IP or DHCP resevation for the syslog server?

  @hspindel 

I had a problem like that, too. I was sending, and still do, syslog messages from several devices in my network to one of my Synology NASes. There were two TP-Link APs among them. Everything was fine for a couple of months, but one day syslog messages from one of those APs stopped coming. I have never found out why. I even tried to send them to another Synology NAS, but it did not work either. Now, when I think about it, I suspect that it might’ve been related to the Omada Controller I installed on my Linux server about the same time. However I were not using the Controller at that time at all. I did not adopt any devices into it for at least three weeks and I did not make any changes to its default configuration. I certainly did not set it up to send syslog messages at that time. I was still managing the APs with their Web Admin portals. Now, I have both APs adopted into the Controller and its syslog messages sent to the NAS so I’m not trying to solve that mystery anymore.