This for some reason occurs mostly during PvP for me. Tho i did have this issue at the Kzarka Quest a couple times too. during normal grinding sessions i never get this. 

I sadly cannot tell what exactly causes it. but what ive noticed is that i get it alot in Battlearena. or as said i had this at Kzarka spawn or Rbf alot.
I could most likely recreate it again by doing some duels in BA

It cannot be my Keyboard as this has been swapped before and it still occured. 

In the Video i have literally just respawned in Rbf and got this bug again. unable to use skills properly in pre awakening, only mashing buttons makes them work sometimes
later in the video i show how everything should behave normally. 
only going into character selec or leaving BA/Rbf fixes is usually. so maybe it has to do with the location? which is very ominous