If the sentence is taken literally, either interpretation could hold. However, for most readers or listeners, only the second interpretation is ever thought of. That is, all but a tiny fraction of auditors take the meaning to be as below, where X denotes the thing in question which is not yet proved or disproved:
We believe X at present, and will continue to do so, until such time (if any) as X is disproved.
Under the second interpretation, there is no implication that X will or might be proved wrong, but such proof is not thought impossible. The first interpretation, on the other hand, seems to require belief that X will be proved wrong. But if you know or believe X will be proved wrong, it is willful perversity to meanwhile believe in it; so apparently the first interpretation is unlikely to ever be correct.