Maybe a stupid question, but extremely annoying.
I can't seem to find any button or means to reply on most existing threads in the forum.
Some threads I can reply on, it show a button under the message, but most threads don't have this button.
Anybody else having this problem?
Is there a logic behind this?

Got an example of a thread you can't answer to?
Are you using any script blockers in your browser?

For example this one: 'Why doesn't Boinc schedule earlier deadlines first?'

https://boinc.berkeley.edu/forum_thread.php?id=13359

There is no button in that one.

Using just vanilla Chrome on Windows and Firefox on Linux. Both have the same behaviour.

Edge on Windows same behavior.

But I can see the reply button on my own thread, which makes me wonder if a script blocked by the browser would make that distinction.

That thread is locked. Only moderators can answer to that thread.

When looking in the index and the thread is preceded by this image, it means it's locked.

So I see. It wasn't clear as I landed on that thread by searching, not through the index.
So almost all threads are blocked? What is the policy/thinking behind this?
Is there a reason to block so many threads?

Mainly to anticipate spammers who like to necromance old threads to piggy-back onto, but also to stop users for adding their stuff onto older threads. We have a 90 day auto-lock feature. Is a thread not posted to in 90 days, it's automatically locked. And we'd rather you post your own problem in your own words, than we'd have to wade through potentially hundreds of posts from 2005 to get to yours in a thread, only because you wanted to pull up an ancient thread to add to.

Some of these threads have been locked by a moderator because they went so far off topic that the end was a flaming paradise.