Try connecting the transformer directly across the 380v supply without any of the capacitors and measure the secondary voltage.
Then try it with the two capacitors fitted.
Those capacitors look very suspicious, and are large enough to create a resonance, but the effects of that are difficult to guess, which is why some experimentation may be in order.
Is this a straight conventional transformer, or does it have a shunt magnetic air gap (like a neon sign transformer) ?