It seems very tough to locate a wrench that is thin enough to fit between the adaptor head and the crank.
Unfortunately here in Singapore my only source is not huge hardware supermarkets, but smallish shops packed to the gills with DIY stuff. They have mostly basic equipment, but after visiting 3 stores, all of them don't have the wrench required to access the thread (head?) so I can loosen it.
I honestly can't remember what I did to tighten the adaptor - especially without using the proper tools - to that point, but now it seems too tight to remove without using a slim-profile wrench.
I did forget to add a washer before installing the EZY adaptor, but I'm not sure it would have made a lot of difference.