I found the problem by a lucky chance:

In our client's environment we didn't set the role name in our config
file, because we do not use roles anymore.

In our local environment the role name was still in our config.

Our code has the following line:
fbwds.setRoleName(role);

The exception gets thrown if role is null. If it is not null,
everything works okay in both environments.

Cheers,
John