Why am I getting a red tint in the lock screen in Oreo 8.1/Nexus 5X? I just updated to Oreo 8.1 from 8.0 and I notice that the lock screen has the same image as before, but with a red tint over it, like a layer of pure red with opacity 20% had been overlaid on top of it.

I never got that in 8.0 or earlier.

I thought it was due to a certificate from AdGuard and the resulting "Your network may be monitored", so I uninstalled the certificate, but the warning disappeared while the tint stayed (I have to say I haven't rebooted the phone without certificate).

I didn't change any setting about system colours, and in fact the rest of the apps and system show the usual colours, only the lock screen changed.

What are the possible causes of the red tint in the lockscreen in Oreo 8.1?
Google support says that the tint is based on the colour of wall paper and there is no way turn it off. That's for Pixel phones and perhaps same applies for yours

Like you said the color tints are based on your wallpaper. I've found it typically does a nice job with this sorry you don't like it. There is not a way to turn this off.

Edit The colour tint applies to lockscreen too going by Android Police blog

Lock screen
It looks like the lock screen does this as well. However, the effect is even more subtle there.