No trick that I know of. You could try refreshing, by clicking these links while logged in:
https://www.gog.com/forum/myrecentposts/refresh
and here
https://www.gog.com/account/refresh

Does clicking on the old Grimrock forum make any change:
http://www.gog.com/en/forum/legend_of_grimrock/

It could be a glitch which only support can fix. Should be a simple flag to delete. I'd definitely send a ticket.

Thanks but none of the 2 refresh worked and the old link is already the link linked. Have constantly the red warning is tedious, I never bothered check it but now I want try use it it's a nuisance. So I'll try contact support.

Log out of GOG, delete any GOG cookies from your browser, restart the browser and log back in. That used to fix stuck notifications in the past, might still work now.

Didn't work. Weird, it worked, thank you, honestly that's rather bizarre for me that it worked. :-)

It is the same post, after all, just that the URL has changed since you got the reply and the notification.

Perhaps but nothing showed in the link it was the same post, in fact the link was a thread not a specific post.

I wonder if a thread with the same name in any forum would have work, anyway it was very bizarre. Perhaps the manager keep a trace of links changes but it's not reflected in user profile.

There's a small possibility the real solution was to remove the cookies but worked in two times (delete gog cookies and connect to gog didn't solved it).

Right, I should have said "thread", not "post".

As soon as you visit any thread, GOG marks all the posts therein as read by you (or perhaps marks the thread as read at <timestamp>, that would probably be more efficient) - something you see as the thread link changing colour (on the light theme they become a lighter gray rather than the almost-black unread threads have), and as soon as the thread gets a new post you haven't read, the thread link becomes "unread" again.

If the new post is a reply to one of yours, you get a notification and the notification stays until you go visit that thread (or, depending on how exactly it works, "until the post that's a reply to yours has been 'read' by you").

I would assume -from your description of the problem- that the notification's URL is set when the reply is posted, rather than being built when you check the notification, thus the forum name change made the link not work.

But no, it wouldn't work with "any thread by the same name in any forum". Most likely, all threads have an ID that's unique across all forums (which is not visible to us mere visitors), and it is by visiting the thread of that ID that the notification is removed.

One notable (and annoying) exception: If the first post in a thread is downrepped to the point of being "low rated", the thread link will always appear read, even if there is a new post. As you can see, this is extremely annoying when it happens to a thread you are actually interested in.

Yeah, it would probably be better if there were different ratings for the opening post and the thread itself.