Reported upstream in Quarkus issue #33218:

When I put this in application.properties:

quarkus.log.console.format=%d{HH:mm:ss.SSS} %clr(${LOG_LEVEL_PATTERN:%5p}) %blue([%-15.15t]) %m%n

I get an error message that lacks information:

Caused by: java.lang.IllegalArgumentException: Encountered an unknown format character
        at org.jboss.logmanager.formatters.FormatStringParser.getSteps(FormatStringParser.java:204)
        at org.jboss.logmanager.formatters.PatternFormatter.setPattern(PatternFormatter.java:76)

(...)

The problem is in org.jboss.logmanager.formatters.FormatStringParser:

                    default: {
                        throw new IllegalArgumentException("Encountered an unknown format character");
                    }

should be something like:

                    default: {
                        throw new IllegalArgumentException("Encountered an unknown format character (%" + formatChar  + ")");
                    }