Sometime it is impossible to click the small top-right "edit" button on a record, even if it work for the other records of the same collection. I suppose that I inadvertently activated some kind of lock while updating it heavily. 

I would just like to clarify this to generally use better practice. 

 I noticed the same issue.
It seems to happen randomly after frequently modifying the same record manually.
The only way that seems to fix it, is copying, removing and re-inserting the record.
This method doesn't work for the player collection though, because after that, the player can't login anymore.

It seems to occur mostly when a field is made junk through cloud code in a way that the editor would not have permitted me to save the record in this state. It's a bit frustrating to have to do the re-inserting instead of just correcting the mistake directly but one gets around. 

Never happened to me in the player collection though. :/

I found the problem on our side.
In cloud code I did a wrong calculation and a number got stored as NaN.

As soon as the JSON file contains one NaN, it is no longer directly editable in the NoSQL section.