Hi!
Just to straighten some terminology and setup out here.
You have IAP-205s in the ceiling plugged into PoE uplink switches (these are mesh portals, right?)
You have IAP-205H (mesh points) that will use the mesh link to ceiling APs as uplink to the network. The wired ports of the IAP-205H have been configured to provide some kind of VLAN to a downstream switch but when that port is plugged in the IAP-205H reboots and doesn´t come back up?
Are all these APs one big Instant cluster? How big is it?
What Instant version are you running?
How is the downstream port of the IAP205H configured?
If it´s an accessport, can you successfully connect a PC to it and get network access?
When thinking about it, it´s not that you´re plugging the switch in to Ethernet 0 and ethernet 0 has a higher uplink priority than the mesh? Try using Ethernet 1 instead and see if it works better.
Cheers,