User:Misibacsi reported that Mark all changes as read does not work and mark changes as read, no matter how long the user waits. You can watch it here.
It is strange because it works if I use it, so there are the user's JS and CSS subpages just in case: link.
Can Misibacsi please try again with safemode=1? This allows you to test if a problem is because of your user scripts or personal gadgets without uninstalling them. See https://www.mediawiki.org/wiki/Help:Locating_broken_scripts for more information.
We tried it with different browser (Firefox and Chrome), disabled add-ons and deleted JS/CSS pages. Other user reported the problem too, so something is wrong and should be fixed.
Does the problem also happen when using safemode=1 on Special:Watchlist?
If it does, please provide any non-default settings on https://hu.wikipedia.org/wiki/Speciális:Beállításaim#mw-prefsection-watchlist to reproduce.
I cannot reproduce with my watchlist on https://hu.wikipedia.org/w/index.php?hidepreviousrevisions=1&hidecategorization=1&hideWikibase=1&limit=250&days=30&title=Speciális:Figyelőlistám&debug=true&urlversion=2 :
The button Összes változtatás megjelölése olvasottként is active and can be clicked here.
Using Firefox 62 on Fedora.
@Aklapper Per reports of the affected users, the problem has been resolved this week, which means somebody fixed it (by accident?).
Thanks for the update! Closing as declined as nobody cannot reproduce anymore and as it's unclear what was the reason / fix.