I think I can figure out what happened.
Although our host system time was set right to GMT, maybe databases server had a bad setting. So when system admin puts the clock back, phpbb forum turns "confused" and it had problems with posting during about four hours (the time was four hours fast).
Some new posts, like chronoh's one, was added on the past (back to the future!

)
Now it seems all are ok.Touch wood!
Luis.