SYMPTOM:
An empty XML-API log file appears in Strategy Web log path when XML-APPI logger is enabled in Strategy Web 9.x, as shown in the image below;

CAUSE:
A possible cause of the issue is that the user ID is entered in "User name" field in Web Administrator > Diagnostics > Configuration > Advanced Logger Options, as shown in the image below:

ACTION:
In "User name" field, enter the full name of the Web user instead of the user ID, this will log messages that are generated specifically by this user, as shown in the image below:

For more information regarding Strategy Web XML-API log, refer to the following technical note: