For example: If I have four learning objects (25% each) and one of them is a poll, I can only complete until 75% even after filling out the poll. By this, I am not able to close the course.
Is there a way to "fix" this or make the poll count in the progress bar?
this behavior only happens if you complete the poll as a teacher/administrator. And it happens for a reason, i.e. not to make the teachers vote for themselves in polls. Polls are anonymous and their results are aggregated, so you wouldn't know it and the results would be "dirty".
Try to complete a poll as a student and you'll see that it correctly completes.