since you have 'white stuff' around the threads-ie scaling
i dont know if you need to do this now, but it might be prudent
id suggest that you use either a commercial scale remover or white vinegar
-soak some cotton wad or tissue and allow it to dampen the threaded areas for about an hour, and then brush off with a childs toothbrush -or whatever may fit the inside of the filler to alow you access to the threads-even pipe cleaner with the extra stiff bristles would do the job i think
you can follow this up with a thin coat of the penetrating oil, again worked with a suitable stiff brush , and several insertions and removals of the piece , wipe again, and then remove as much as you can of the remaining film-
and then
using 1:3 or so white vinegar to water-light er up and use the acidified water to descale inside the boiler
youll enjoy the tangy aroma of salad as well!!!LOL
id also suggest that for the next few steams-leave the cap off and allow it (and the boiler and its threads,) to dry-the repeat insertion etc, will also help to remove any remaing stuff, and should reduce the chances of it seizing up again