'An error was detected in the current theme that could interfere with the system login process'

[diffficult]

Getting a dialog across the screen with that message and the options 'Load Default Theme', 'Load Fallback Theme', 'Cancel' after a cold boot.

'Load Default Theme' gets me to a standard default theme and works fine. Pressing 'Cancel' a couple of times makes the dialog to go away and doto works just as intended.

If I log out from a working session the dialog doesn't come back. It only appears when rebooting or starting up the machine from a shutdown.

[devbridie]

Same here, unfortunately. Using version 2.2.5-1 of lightdm-webkit2-greeter.
When using lightdm-webkit2-greeter to run the theme, there are no errors in the console, but the video does freeze up after just a few seconds of displaying.
When lightdm uses it normally, I get the same message as @diffficult. There are no relevant entries in the greeter logs that I can find with debug-mode on.

[devbridie]

I suspect a regression in lightdm-webkit2-greeter. Using 9a433a3 results in the same error, but it is the same version as I had been using on another PC where the doto theme did work. Unfortunately I am not able to find out which version of lightdm-webkit2-greeter the other PC used.

[mr-remington]

I get the same error. I managed to at least supress it without visible breakage by setting:

/etc/lightdm/lightdm-webkit2-greeter.conf:

[greeter]
detect_theme_errors = false