Since updating to Jenkins 2.50 ANT builds that use environment variables fail on windows slaves because lots of environment variables are now all lowercase.
Using environment variables in ANT is case sensitive.
Variables that come from the system are all lowercase (COMPUTERNAME, Path, SystemDrive, SERVICE_ID, ...) and variables that come from Jenkins are as usual (NODE_NAME, BUILD_ID, ...).
SERVICE_ID is interesting because it is not set on the system, used to be all uppercase is now also all lowercase.
SSH Slaves are not affected. Windows slaves use Java Web Start and jenkins-slave.exe as a service.
Researching I found this which may be related: