I've been having the same issue as well starting I think Tuesday of this week. I'm using an xbox 360 controller from microsoft. The controller switches to player 2 on it's own (even when the controller is not in my hand) and none of these solutions fix the issue: switching it back to player 1, unplugging and plugging it back in, or plugging in new controllers. Sometimes restarting PSO2 solves the issue and sometimes I have to restart my entire computer. To verify that this issue was only on PSO2 I minimized the game one of the times of this happened and launched Tales of Berseria. The controller functioned properly with Tales of Berseria, but as soon as I swapped windows to PSO2 it stopped working.

Same issue. RIP items.