A couple of observations what happens at my computers:
- silverlight is not necessary on the forum, the forum works fine on both linux and macbook, both lacking silverlight which is windows only
- flash not necessary as well, although it gives you some extra features like multi upload. But the forum runs great without it.
- java is not used on the site afaik. Didn't install java on my minimal linux installation and the forum runs fine on that one although without the extra features because lack of flash. Javascript is enough.
- not able to start a threat happens to me as wel in all browsers ( firefox, edge, chromium, opera, safari ) on all systems ( osx, windows and linux )
- posting threats start working for me after removing all cache by commandline on linux or sometimes just after refreshing.
So for me it seems like a problem with the cache, no matter what browser or os. Take note that opera, safari and chromium (opensource chrome) all use webkit as render engine, while Firefox has gecko and edge has it's own engine so the problem is fully independent of rendering engine.
Of course these are my bservations, only on my computers, so no garrantee that it is the same for all users.