Today we updated this forum and the underlying database to the latest versions (nodeBB is now on v1.16.2) for security and bugfixes.
Unfortunately this introduced an ugly bug where you can no longer mark all unread messages as read.
We raised an issue to the theme developer. We can't do more than that currently. Sorry.
You can follow the issue on Github: https://github.com/pichalite/nodebb-theme-slick/issues/51