Depending on AP firmware version and the capabilities of the haldheld client, you may need to try out decreasing the minimum bit rate to something less than 12Mbps (try 11Mbps) and/or disable 802.11r on the Wireless > Access Control page for that SSID in case the client doesn't support 802.11r and try that as well.
Well, this looks like more like a connectivity issue. Can you describe the settings you have on Wireless > Access control? If you have a password there is no need to post it here. What I would like to see what settings you use to make a recommendation based on that.
How far away are the end devices from the Wireless APs?
sorry for the delay answer. The end device is 1 m from AP.
We problem with MR33, MR66, MR74 and we think that is problem with the chip in the AP.
Now we have a dedicate TAC Ingenieur. I hope we can fixed.
Problems with old tech scanners is widespread, across all sorts of networks.
Put them on their own Wi-Fi network that has the oldest WiFi standards enabled (802.11a and 802.11b) and the original (weak) password security, and turn off anything remotely clever.