Ubuntu 18.04 wifi doesn't work after update

I'm running Ubuntu 18.04 on a Dell Precision M6400. When first installed 18.04, WiFi was running fine. After automatic update, it doesn't any more. WLan spots are displayed correctly, but when trying to connect it fails after about 1 min, a message appears: Connection failed. Updated and upgraded already, but no success. Looks, like the driver / firmware is no more correct after update. Any hints?

$ lspci -knn | grep -i net -A3
09:00.0 Ethernet controller [0200]: Broadcom Limited NetXtreme BCM5761e Gigabit Ethernet PCIe [14e4:1680] (rev 10)
    Subsystem: Dell NetXtreme BCM5761e Gigabit Ethernet PCIe [1028:0251]
    Kernel driver in use: tg3
    Kernel modules: tg3
0c:00.0 Network controller [0280]: Intel Corporation WiFi Link 5100 [8086:4232]
    Subsystem: Intel Corporation WiFi Link 5100 AGN [8086:1321]
    Kernel driver in use: iwlwifi
    Kernel modules: iwlwifi

--

$ rfkill list
0: phy0: Wireless LAN
    Soft blocked: no
    Hard blocked: no
1: dell-wifi: Wireless LAN
    Soft blocked: no
    Hard blocked: no
2: dell-bluetooth: Bluetooth
    Soft blocked: no
    Hard blocked: no
3: hci0: Bluetooth
    Soft blocked: no
    Hard blocked: no

$ uname -r
4.15.0-33-generic

After switching back to kernel 4.15.0.32.35 it doesn't detect the wifi adapter resp. doesn't start up the driver iwlwifi.

$ lspci -knn | grep -i net -A3
09:00.0 Ethernet controller [0200]: Broadcom Limited NetXtreme BCM5761e
Gigabit Ethernet PCIe [14e4:1680] (rev 10)
    Subsystem: Dell NetXtreme BCM5761e Gigabit Ethernet PCIe [1028:0251]
    Kernel driver in use: tg3
    Kernel modules: tg3
0c:00.0 Network controller [0280]: Intel Corporation WiFi Link 5100 [8086:4232]
    Subsystem: Intel Corporation WiFi Link 5100 AGN [8086:1321]

$ rfkill list
$