It is possible to have a "good" strong signal, but not be able to pass traffic ..... because the signal quality is bad.
The SQ could be bad from a variety of interfering signals or multipath.
The error you've posted would be the same as if the client suddenly lost power .... the client just went away ... (or didn't respond, so the AP cleared the association).
You may want to check your NIC settings; some drivers/utilities default to powering down if there's no traffic for some period of time ('Power Save mode").
This is different than the wireless power save mode where the AP knows the client is sleeping and saves/stores the traffic until the client "wakes up" (typically a few milliseconds).
Some BIOS have setting to allow the system to put the NIC to sleep after a period of inactiviity as well.
You're either losing the signal (strength and / or quality), or the NIC is going to sleep (by the OS or the BIOS).
Good Luck
Scott