If for some reason renaming file fails (see http://logback.qos.ch/codes.html#renamingError) then Logback internally throws NPE
See stacktrace.txt file attached.

Reason of heaving this exception: We are trying to rename existing file with path (lets say 'c:\Temp\2\log.log') to a file with just a name (lets say 'log2.log')
In this case if renaming fails for some reason then we are trying to compare volumes of existing file ('c:\Temp\2\log.log') and a parent of a target file ('log2.log).
Parent is null in this case.

Fix would be: RenameUtil.areOnDifferentVolumes should return false if parent file is null (patch file attached)