You have to enable the KWallet Password extension from Preferences > Extensions
You will be asked for the KWallet password and you should have it there. Hope this helps!
EDIT: So, it seems the issue is totally different than previously described. It works on different sites but not on this forum. Check if the built in Password Manager has the AutoFill options enabled (both options), if is set to store in the KWallet backend, and has no saved password from previous backends and also if there is something to the Exceptions list.
Log out from the forum, clear the cache and all the stored data, close the browser and reopen it and try that way.