"This board has been updated" when only one user doing changes

[Pascal Robert]

Hi,

We found a weird issues where an user would get the "This board has been updated" dialog while moving 5 to 6 cards in a Kanban board, in the same column. I can replicated this with my (admin) user, but I have to move about 15 cards to get it.

But the main problem is that sometimes, the cards go back to their previous rank, which is bad. 

Problem is present on 8.20.15, and was also present in 8.4.2. I don't see any HTTP errors in the developer tools.