I am having the same problem with a mini pc that came with windows 10 ans i installed linux mint xfce, but in every boot i have the problem and the same dmesg output as your regarding iwlwifi and not working wifi. Had you reached any solution?
This is dmesg
[ 6.862754] Loading modules backported from iwlwifi
[ 6.862756] iwlwifi-stack-public:master:9858:4c7cba27
[ 7.526764] iwlwifi 0000:02:00.0: can't change power state from D3cold to D0 (config space inaccessible)
[ 7.526774] iwlwifi 0000:02:00.0: can't change power state from D3cold to D0 (config space inaccessible)
[ 7.526963] iwlwifi 0000:02:00.0: HW_REV=0xFFFFFFFF, PCI issues?
[ 7.527769] iwlwifi: probe of 0000:02:00.0 failed with error -5
Might not be fully related to the problem described but it came the first when I was googling my problem.
For me, the error was coming up every time I suspended my machine but was working fine when booting from the off state. After days of trying different things it turned out to be a problem with Nvidia HDMI Audio controller. The below solution worked for me!