My dictionary says that' sonraki' can mean 'following' in which case perhaps the sentence can be translated:
A following conference is in that hotel. This seems to get over 'bir' being translated as 'the'
Quite confusing, but it is so and we just have to remember it. Perhaps we have to consider that sonraki in an adjective and that bir meaning "a" is always between the adjective and the noun. Thanks for your explanation.