We have a situation that may cause a few cases to be raised with Microsoft support via partner channel. After upgrade to BC18, users can no longer log in, if they use NavUserPassword. Password needs to be re-created. This will be resolved in Wave 2 2021 and not in a cumulative update. In the meantime it got documented here, along with a possible workaround:
Known Issues with On-premises - Business Central | Microsoft Docs NavUserPassword authentication doesn't work after upgrade to version 18
And there is a Yammer thread too (requires that partner is signed up for Yammer):
(166) Yammer : Dynamics 365 Business Central Development : View Conversation
The customer can at least get unblocked quickly by suggestions provided