Questions that lend themselves to be formulated in a concise and general way can be made into a Q&A topic, where both developers and advanced users can weigh in their answers, and where they may be of benefit to others encountering the same issue. Be sure to check first if your issue was not discussed already!
If you believe that you have spotted a bug, the best thing to do is to file an issue. Issues should contain a description of the problem, the expected behaviour, and steps to reproduce, including the version of Nutils that the issue relates to. If you believe that the bug was recently introduced you can help the developers by identifying the first failing commit, for instance using bisection.
Finally, for general discussions, questions, suggestions, or just to say hello, everybody is welcome to join the nutils-users support channel at #nutils-users:matrix.org. Note that you will need to create an account at any Matrix server in order to join this channel.