If I try to add a new tag to a question a colleague has created, this first seem to work, but after saving an error message is displayed briefly: "Access denied".
How to allow this? We all want to improve other questions.
See with your admin if your instance has reputation-based permissions enabled. That means when you accrue a certain number of points, you are able to do certain admin functions like update another user's question's topics. Otherwise, only admin can do it.
I haven't found any explicit documentation about editing other people topics but I've tested in my instance and it seems only users that are presented in the administrators group can edit topics in other people's questions. I believe you could do this by editing questions permissions or even the space permissions, but again I haven't found any documentation indicating which permission controls this.
Hope this helps.