EDIT: I stand corrected. See other answers for details.
TL;DR - It means the latter. However, it looks like some Japanese confuse the meaning.
I personally was confused with this phrase. This is because "開放" was accompanied with "厳禁" (strictly prohibited.) This raised a feeling that opening it would get me in trouble.
One of the results in my googling shows me that "開放厳禁" indeed means both. However, from the point of the meaning of each word, it should not mean the former.
I looked for "開放" in dictionaries and found that "開放" means "to leave something open" (It does not mean "to open something." The entry is found in デジタル大辞泉.) Hence the meaning is the latter - "Strictly don't leave it open unless in the case of emergency."
I did a further search on other Q&A sites and found that if the person who put the sign really didn't want someone to open that door, it should've said "締切," which means "Keep it (like windows or doors) shut," or "立ち入り禁止."
P.S. I went through the door after that because it didn't have any security measure (something like in naruto's answer.) I asked this because I saw that door sometimes used by somebody who doesn't seem to be a worker there.