It is Volume Up and Power for only 3 seconds to reboot Xperia X. The device vibrates ones. To power off keep holding and wait for triple vibrate.
I need it way to often sadly.

Sometimes instead of the lock screen i only get backlighted black screen with no notification led error code.
Most propably i misconfigured my installation due to testing all and every hack i read here. If the XA2 port does not supercede the Xperia X ux, i am propably trying a reflash and keep everyting stockish to see if the error goes away.