Release notes (RNs) summarize main changes between two consecutive software versions and serve as a central source of information when users upgrade software. While producing high quality RNs can be hard and poses a variety of challenges to developers, a comprehensive empirical understanding on these challenges is still lacking. In this paper, we bridge this knowledge gap by manually analyzing 1731 latest GitHub issues to build a comprehensive taxonomy of RN issues with four dimensions:\textit{Content}, \textit{Presentation}, \textit{Accessibility}, and \textit{Production}. Among these issues, nearly half (48.47%) of them focus on Production; \textit{Content}, \textit{Accessibility}, and \textit{Presentation} take 25.61%, and 17.65%, and 8.27%, respectively. We find that: 1) RN producers are more likely to miss information than to include incorrect information, especially for breaking changes; 2) improper layout may bury important information leading to user confusion; 3) many users find RNs inaccessible due to link deterioration, lack of notification, and obfuscate RN locations; 4) automating and regulating RN production remain challenging despite the great needs of RN producers. Our taxonomy can serve as a roadmap to improve RN production in practice and also reveal interesting future research directions.