Please read the release notes in the news forum.

This is not a bug, the building levels have been updated.
Most of them now only have 3 levels instead of 5. (Giving the same max bonus).

This means: If you had level 4 before, you will now have a half finished level 3...

This corresponds to the building you had before and will give you the same bonus on production you had before.

I'll make this sticky for now, since people will probably repeat this question.

thank you... was reading at updated list when i saw this one and said: "OH"

Hmmm... Barrack levels were not affected by the update. I mean, maximum level is now lv 3, but all my barracks are unchanged in level (and so all my lv 3 barracks are now at max level instead of a damaged lv 2 which would be the corect conversion)
Also think the same happend with naval bases, but not sure as i had only few that were higher than lv 1 and can't remember their level before the update but believe it was not affected

Naval Bases were affected - I had a level 3 Naval Base (which was building a BB) downgrade itself to level 2.5.

naval bases and infrastructure is what I have noticed... hopefully this wont be an issue in games started after the update.