With a 3W load it goes worst, with peaks at 17 V.
With 10uF in parallel on the load it improves, but still a lot of spikes (+-1V).
Sure it is a low quality Power supply. It was working fine earlier, powering an arduino. The the Arduino got mad, I checked the PSU and found it guilty.
Seems like a fault.
Several years ago I purchased 10 pcs of this power supplies from ebay, and they all fail after 1-2 years of operation.
The curious thing is that those "blocks" of spikes are aligned with the peaks of the 230V supply voltage.
Maybe someone recognise that "shape" of the signal as a "typical" fault?