How do I Unsolve a Question that I marked as solved by mistake

It is neither, it is the way it is designed to operate. On the scale this site operates people going back and forth adding solves and removing them would be a tremendous nuisance plus keeping track of the points removed/added. There are enough real bugs here without adding meaningless complexity for something that doesn't occur that often.

Sorry you made a mistake but as you have been repeatedly told you CANNOT change it. Period.

If you want people to continue to answer you can add a post saying you accidentally clicked the button, that post does not solve your problem and you are still looking for help. Or you can create a new thread and this time not make a mistake. Your choice. But you cannot change it.

Post a followup that indicates the marked solution did not solve the problem.

gereonb wrote:

The post marked as a solution is not mine so there would be no way I can edit it. I can also not edit my original question so as far I can see there is no way to unmark the Question as "Solved".

i.e. my original question remains unsolved.

Correct. There is no way to change a post marked solved to unsolved. If you marked the answer solved, post a reply to it indicating you marked it solved in error. If it is someone else's post that was marked solved you can not change their accepted solution, and why would you want to?

No, post the followup in the original post where you incorrectly marked it. And there is no way to change it.

You still aren't getting it, ADD a post to that thread saying just what you said here, that you mistakenly marked that post as solved and your question remains unsolved.