How to mark as solved?
There is an on-going discussion in the moderator's part. The platform allows to "Ask question" or to discuss ("Submit"). The problem is in what is default. When a question has been as asked, everybody answering will see "ask a question" for submit, which is really confusing, because they are actually answering.
The other option is to have discussions as default and everybody sees "Submit", but the "Mask as Solved" under "Topic Tools" vanishes.
In order to mark discussions as "Solved" go to "Topic Tools" and press "Ask as question". Open "Topic Tools" again and now you should see "Mark as Solved" to press.