Wireless bridge eating away frames

Hi folks,

I am having a weird issue with one network. A print server losses connection to the printer if it is over a wireless bridge. But if the server is running a constant ping to the printer, it works no problem. It looks like the following:

|Print-Server|-----|Switched-network|-----|AP| ))) |AP|-----|Switch|-----|Printer|

I mirrored the respective access ports on both ends and sniffed the traffic from the server and the printer. Somehow, the TCP packages leave the server but never arrive to the printer if the ping is not active (it also works for the first few seconds after we turn off the ping).

We have also tested it without the WLAN-bridge and all the packages arrive well. It is obviously something in the bridge causing the problem, but I have not idea what to look for. Other services/host connected to the remote switch do work without an issue. I would like to sniff the APs' ports as next step, but hadn't had the change yet.

We are using Unify Switches and APs with some old Cisco switches that will be replaced soon. We are able to replicate the issue in a lab environment with just 2 switches and 2 APs. If we remove the APs, the problem goes away.

Do you guys have any idea what may be causing the problem? Or what I am actually looking at here?

Thank you very much in advance.