Bangle.js 2 button stuck

Ok, yes, the lowered resistance sounds like it then :(

Lets suppose I would try to open it, how or what could I check and/or repair inside?

There's a little button on a flex PCB - and either you could just give it a clean under distilled water, or I do have a handful of new ones here and I could send you one of those.

edit: Just to say that it is possible to stop the button resetting the watch (https://www.espruino.com/Bangle.js+Butto­n+Reset) so even if the button stayed stuck down sometimes, you could almost certainly still use the Bangle for something even if not having the button available made it not that good as a watch.