Recently I’ve started to see this message on odd occasions, it seems quite random.
Your submission could not be processed because a security token was invalid.
If this occurred unexpectedly, please inform the administrator and describe the action you performed before you received this error.
It happened a few minutes ago after clicking the “Mark Forums Read” link, but I’ve also experienced it when posting or previewing a message. A second attempt has (so far) always succeeded.
Thanks for reporting. What browser were you using?
I haven’t seen that one. But I do occasionally see things go awry.
Firefox ESR 52.9.0-2.97 ( from http://download.opensuse.org/repositories/mozilla/openSUSE_Tumbleweed/ )
I’m using firefox, but the esr version from Leap (currently 60.5.1). And yes, I liked 52.x better than 60.x.
When things fail here, I usually get a message about a corrupted page.
What happens: you connect to openSUSE. And if the opensuse cookie has expired, you are redirected to a microfocus login page. And from there, you are redirected back to the openSUSE forum page. If something goes wrong in this chain of redirection, you run into problems such as you are seeing. And a second attempt will often work.
Much of this might change soon, with SUSE and openSUSE becoming independent of microfocus.
I am always using the latest version of Firefox (currently 66.0) and never experienced problems with the forum. Thus I prefer everything stays the same. Login could be somewhat faster with SUSE becoming independent.