I've not seen that error log before. But you can confirm your max arp limits by running.
get sys-cfg
And see the current usage along with the max on the device at any moment with
get arp
So if you can run this when the sys log occurs it would verify if you were at or over the arp table limit.
The other thing I've seen the Q used for as a symbol in ScreenOS is vlan tagging so it might be related to that in some way too.