I have not investigated this thoroughly, but there is definitely something wrong with enabling/disabling checkboxes in finished cards by mouse clicking. Here is a video.
And here it is how the card looks in Edit mode:
Can you reproduce this? (Big Sur, Safari)
Hi @Valery_Kondakoff, this is very strange. Do you mind pasting the contents of the problematic card here on the forum or send it to us via the in-app messenger? From there I will investigate and try to find the root of the problem.
@Tobias, of course, I can. Maybe it is better idea just to share the card with you?
Hi @Valery_Kondakoff, thanks for sharing the card with me! I figured out the problem. It is to do with inconsistencies with the case - using lowercase x in the checkboxes rather than an uppercase X creates this issue.
We will look into fixing this in the next update, but a temp fix for now is just to stick to using a capital X!
@Tobias, great! Thanks for looking into this.
@Connor: changing the case of many [x] to [X] is another use case for search and replace. 
@Valery_Kondakoff Haha fair point.
Fixed in 1.4.8.
