I think it would be beneficial to send out a newsletter on forum use, create a "how to post" stickied, use social media a few times to link to it.
Then finally in the emails ask people to respond by email if they can't get forum access.
Until a few weeks ago I didn't have the login to post or have any idea what these changes were. I wouldn't be surprised if a lot of people don't also have similar problems, so it will take some effort to help everyone get on board.
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Questions on how to use the forum? 3D Design & 3D printing? Please ask me!