Hello @janisK,
A fix for the missing ACF Groups has been issued in 1.2.8.
If you still encounter issues with this specific bug, please let me know by replying to this thread.
As for a “simple” way to add a lightbox to an image, we have implemented a start to this feature in the Image block → Primary Tab → Settings
Hopefully that helps quicken the workflow. We will be adding all lightbox properties to allow you to take full advantage of it in the coming weeks.
Cheers,