First of all, I was unable to surely tighten down the problem to the E90 firmware, but the problem probably relies in it.

My company is using building-wide wireless network with both unencrypted and WPA-encrypted network. Both networks are served by Meraki Mini APs running OpenWrt Kamikaze 7.09, the separate SSIDs are bridged into separate VLANs terminated on a firewall, like this:

SSID1 - VLAN2 - firewall VLAN2
Meraki AP
SSID2 - VLAN3 - firewall VLAN3

Using 7.24 firmware, both networks worked fine. However after upgrading to 7.40 (we needed AGPS and so), logging on to SSID2 results in the reboot of the E90s in a few seconds. The problem shows up with both hostapd and hostapd-mini package running on the AP. I'm currently seeking some other S60-based devices with wifi connection to see if those also behave like this.

Testing another network layout with only one WPA-encrypted network and without VLANs, E90s are working perfectly.

If this did catch the eye of an official firmware developer or tester, I'm open to help testing to fix this problem.