Can't connect to wireless network when connecting a 5th device

[Kranenburg]

I've installed the FVS318N router at a small office of a client. Half a year ago, with only a few employees, the wifi worked fine. Currently there are more employees (but still just 5) and we're started having troubles with the wifi. It seems that if a 5th person connects to the network, it can't establish a new wifi connection. If one of the connected users disconnects from the network, the newly arrived employee can connect - but than the other user cannot reconnect anymore. So for the user it looks like there is a (very low) limit on the number of wifi connections.

I've found the following messages in the logs (the correctly working router does not show any of these messages). Obviously, I'm most worried about the first:

[FVS318N][Local0-wireless][UDOT11] umiIoctl(UMI_COMP_KDOT11,2 ,39) failed
[FVS318N][Kernel][KERNEL] Should overwrite TID here for NULL data ntry <0
[FVS318N][Kernel][KERNEL] <sppe_cmd_arp> Fail to delete IPv4 ARP entry <0

I've updated the firmware at both locations (from 4.3.3-2) to 4.3.3-6 - but I did not notice any change.

Any suggestions? Hardware issue? Firmware issue?