Good day! this happened 3 times to me already, I check-in in our guild for my daillies achievement exactly after the daillies reset which is 12mn server time, it so happens that maybe the system of the game is still calculating and it did not complete my daily.

 

Here are some screen shot 

I have had this same issue for 4 or 5 days in a row now.
I waited for reset, checked in, completed task, go to turn it in and it tells me the daily task will be reset soon and refuses to let me claim the reward.
I would like to ask for our rewards please, we need those boss stamps.

Same, I'm not getting flagged as checked in even though the status is clearly checked in.

Perhaps wait for notification directing you to guild if you aren't already.