Diagnosing transformation errors

[urbie4]

Getting an error when I try to run a transform from oXygen Author 23.1:
The transform type is eclipse help. Error text (company-specific stuff redacted) looks like this:

Severity: fatal
Description: Transformation failed. Fatal error during transformation using C:\DITATools\oxygenAuthor\frameworks\dita\DITA-OT\plugins\com.ditatools.base.transform.common\xsl\validate-xsl-override.xsl: Processing terminated by xsl:message at line 99 in output-message.xsl; SystemID: file:/C:/DITATools/DCS/oxygenAuthor/frameworks/ibmdita/DITA-OT/plugins/org.dita.base/xsl/common/output-message.xsl; Line#: 99

My question is not, "What does this error mean," but "where do I find what this type of error message means?" Where's the go-to source for documentation on these errors?
Thanks!