> What am I suppose to do? Debug it ? I don't event know where to start from.

This is the problem. Know that it's mostly a psychological thing. Start by reading the code of the feature you're trying to use. Go through it with a debugger. Yes, the first time it will be hard and take a long time but you will get used to it.

To the manager just say "it's gonna take a little longer than estimated, there's a bug we didn't expect in the library". Eventually, you will deal with these issues so fast it won't even be worth mentioning.