When using the S3 Output plugin with Windows, the UUID has so many characters in the output file name that often the output file name (with tags applied) is too long for Windows OS and the process fails. This happens when applying tags to the output file name. Additionally, the process adds ".partN" to allow for file rolling which adds even more characters to the output file name. Our workaround is to not apply any tags to the output file name. Older versions of the plugin (3.x) used the executing server name instead of a UUID which was far fewer characters when using our server naming conventions. We've has this issue on Windows Server 2012 R2 and Windows 7.
Currently logstash-output-S3 issue #134 is open to allow customization of the output file name by users, but UUID seems like it could be a bigger functional design issue; If the UUID is removed to save on characters then there could be issues with output files overwriting each other.