When you setup the Connection from Dynamics 365 Business Central to Dynamics 365 for Sales by running the Assisted Setup (Dynamics 365 Connection Setup Wizard / Set up Dynamics 365 for Sales connection) it will fail if the password of the Dynamics 365 user (either the integration user or the admin user) contains a space or other special characters.

Using a direct setup instead of the wizard (not recommended as it's not really possible to install the extension on the Sales side) gives the same issues.

The error is 'authentication failed' or in some situations 'bad request'. Changing the password to characters from the azAZ09 prevents the problem.

This should either be documented or changed. Probably the latter as I assume this is simply a bug in some storage field, somewhere (I was able to trigger a 'bad connection string' error at some point).