• Solution # 1
It seems that a restart of the phone will clear the problems, at least for a while. One user reported that changing the theme of his Note 9 through the Samsung Themes application solved his problems. This is highly anecdotal, but it is worth trying if the problem is resolved permanently.
• Solution # 2
As the problem is linked to Samsung Pay, the reconfiguration of those functions should be enough to solve the problem. It seems that some invisible Samsung Pay pop-up window is blocking the keys, and that box disappears once Samsung Pay is set. It's a strange solution, but apparently it works.
Samsung is also working on an update that solves the problem. If you do not want to configure Samsung Pay, you'll have to wait for that update.