I've been running Redmine for over a year now, and truly 100% of the problems users have reported have been this:

They can't work out how to add a new issue or edit something.

In every single case, the problem is simply that they didn't realize that all the editing options are invisible until they sign in, and they didn't see the sign in link in small text way at the top right of the screen.

The ideal solution would be for the "New issue" and "Update" links to be visible always, and to prompt the user to log in when necessary before resuming the requested operation.

If that's technically difficult, a suitable stopgap would be for "New issue" and "Update" links to be visible, that just take the user to the "Sign in" page, without resuming the appropriate operation. At least they could then find their way back to wherever they were and try again, but logged in.

As it is, with literally nothing on the screen to clue them in that they need to log in before they'll see the edit options, and with the login link in such an obscure location, it just never occurs to them that they need to find a way to log in.

Note that forcing sign-in to view anything is not an acceptable solution, because I want the site to be indexable by web crawlers.