I wanted YAML for the log4j2 configuration file because it doffs XML's visual clutter, but had the same error as Guest96. I scoured the web for a solution to the above, investigating a Utf-8 BOM or other content in the YAML header area; no joy. Of course, the answer is usually simple.
Somewhere, I had fully missed it that using YAML with log4j2 required the jackson libraries, per https://www.sentinelone.com/blog/log4j2-configuration-detailed-guide/. Adding the jackson reference to my (Gradle) configuration fixed the problem:
// Gain support for log4j2.// https://mvnrepository.com/artifact/org.apache.logging.log4j/log4jimplementation 'org.apache.logging.log4j:log4j-api:2.14.1'implementation 'org.apache.logging.log4j:log4j-core:2.14.1'// Gain support for YAML with log4j2.// https://www.sentinelone.com/blog/log4j2-configuration-detailed-guide/implementation 'com.fasterxml.jackson.dataformat:jackson-dataformat-yaml:2.10.0'implementation 'com.fasterxml.jackson.core:jackson-databind:2.10.0'With that, the dreaded Fatal Error :1:1: Content is not allowed in prolog error went away.