Hi Ruud,
I think your problem is something to do with Forum site privileges. If I look at your original post when logged in as an administrator, I get a whole series of options in the line alongside the date of the post including ‘edit’. When i log in as an ordinary member, the only option I get is to reply to a post.
I have no idea how to adjust that situation, it’s something the webmaster may be able to investigate. For the moment I think all you can do is reply to your own post to alter/update whatever it was you wanted to edit.
Sorry i cannot be more helpful
PeterF