Apologies if this is PEBKAC

I configured up my appender like this:

<appender name="DS_LOG" class="ch.qos.logback.core.rolling.RollingFileAppender">
<File>/discoveryService.log</File>
<ImmediateFlush>true</ImmediateFlush>

<rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
<FileNamePattern>C:\Program Files (x86)\DS/logs/discovery-%d

{yyyy-MM-dd}.log</FileNamePattern>
</rollingPolicy>

<layout class="ch.qos.logback.classic.PatternLayout">
<Pattern>%date{HH:mm:ss.SSS} %level [%logger] %msg%n%ex{full}%n</Pattern>
</layout>
</appender>

The slightly odd "C:\Program Files (x86)\" is where Windows64 puts 32 bit executable (and it things that Tomcat it). This failed. Due to my own programs limitations it took me a while to get there, but this was because of this statement:

DateTokenConverter dtc = fileNamePattern.getDateTokenConverter();

if (dtc == null) { throw new IllegalStateException("FileNamePattern [" + fileNamePattern.getPattern() + "] does not contain a valid DateToken"); }

when I poked into fileNamePattern.getDateTokenConverter(); I see this:

this FileNamePattern (id=159)
context LoggerContext (id=80)
headTokenConverter LiteralConverter<E> (id=160)
literal "C:\Program Files (x86"
next null
noContextWarning 0 [0x0]
pattern "C:\Program Files (x86)\DS/logs/discovery-%d{yyyy-MM-dd}

.log"

Note that headTokenConverter is malformed, and so we end up with a null dtc.