If all the players reported the same thing as the same time you experienced it then it was most likely a server issue.
Most likely a lag spike.
I don't know what else could've caused it, seeming that it wasn't just you at that time.
Well, yes but this freeze also always happens whether or not any players are online in game or not... So I think it is more of a client sided error?
That doesn't necessarily mean it's a client-side error, it could be a server issue that affects anyone IF they are on the server, doesn't mean more than one person needs to be on.
As Darth said, if everybody experiences it at exactly the same time, it's a server issue, but if they individually experience it whenever they open the menu, it's almost definitely a client-side issue (unless the client is downloading the jutsu list from the server each time you hit 'J', but that seems highly inefficient...).
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