First, I was hesitant to raise this question as I found quite a lot of similar questions asking for help on commenting. It turned out that the reason for these questions often was too low a reputation to comment.
So I first raised the question on the meta site of "Ask Patents" to make sure that it is not a problem with my low reputation. Indeed, I received an answer from a user with over 5000 reps. who could not comment, either. That indicates that it is not a problem with reputation.
Thus I placed this question here because it may be a feature of the stack exchange engine or another policy barring me and some other users from commenting. I might also be a bug :-)
- On (Meta) Ask Patents, the threshold for the "comment everywhere" privilege is set to 1, see the list of privileges of that site.
- The "add comment" link is missing not only on the question but also on answers from other users.
- The "add comment" link appears on my own answer.
- The "edit" link is greyed out and its tooltip says "Suggested edits are not allowed on non-tag-wiki posts on meta sites."
- Screenshot how I see the "question 47" page: