Symptoms different from those in the thread 'Game crashes out of the blue', so I start a different thread for it.
Basically either the client or Java itself hung (beginning of the dungeon floor with Horus on it) without any sort of warning; I could alt-tab to other tasks which were still running, although Flash was also frozen in Firefox (but that is a known and periodic issue, easyly solved).
I then terminated Flash, reloaded a browser page. Flash reloaded as usual, but the client would still not come back to active duty (tried this in case one induced the other, seen weirder things happen)...
Eventually terminated the client, which was at that point found 'not responding' to Task Manager' and took a couple of minutes to be forcibly closed by it.
I then restarted the client, without any further symptoms so far.