@Aklapper: thanks for your feedback
I will try to describe the problem in detail:

• I assume that "thank" is noted locally (browser cache) and not in the database.
• In any case I am logged in.

• I log in and "thank":
• a1: log off and do not close the browser
• a2: I log in again and see "thanked" (OK)
• b1: log off and close the browser (Clear the browser cache!)
• b2: log in again and can "thank" again (not OK)
• c: use a different browser and can "thank" in any case (not OK)

This is a valid issue, but I think it has always worked this way, so I wouldn't say "no longer". :)

When displaying either a "thank" link or a "thanked" message, the Thanks extension only checks a list of your recently sent thanks, which is stored in a cookie "commonswikithanks-thanked" (and similar for other wikis). This disappears if you log in from incognito mode, in another browser, or clear your cookies.

There is a secondary check to prevent duplicate notifications after you click that link, so you can't actually send thanks twice for the same revision or action, even if it looks like it works.

I don't know why it was done this way, but I am guessing that the secondary check is too slow to run it 50 times (or even 500 times), for every revision, when displaying a long history page.

@matmarex: Thanks for the hopeful feedback. (I use the function "thank" often and mostly in incognito mode. That's why I noticed it).