Been reading the forum posts here about the issues with the 7260hmw NIC. I know that there are problems with Win8.1 and 802.1X networks, and others have had to disable "PMK caching" on the client with better performance results. Has anyone tried that yet? Also, is band steering causing the 2.4 authentication process to time-out? Is it deciding to roam and default to 2.4 over 5.8? It looks like the driver is disconnecting itself for some reason, and your server is getting that message.
As far as 2.4 coverage vs 5.8 coverage being about the same, this isn't really true. 2.4 propagation travels twice the distance as 5.8 at the same power output. This is how the physics works. Now, it may seem the same speed but the MCS is higher for 5.8 over 2.4 at the same distance & similar power levels.