I'm sorry for stupidity but what you mean by stuck on [1]?
Anyway, you can try other browsers like Samsung Internet, Vivaldi, brave, kiwi. It might be time to switch your browser
When there is a new message (PM), it will show "My Messages [1]" or the number of unread PM's you have in the forum navbar. When you read the message, the [n] counter should vanish. If it stays, it's most likely an cache/browser (client-side) issue.