The current default is

# Controls when caching is disabled because there is not enough space available on the file system
# Configured value is in BYTES. The default is 1GB
plugin.stash-scm-cache.minimum.free.space=1073741824

The minimum should be raised to at least 5GB to prevent the cache from filling up the disk for big repositories. For instance, if the disk only has 1.5GB free and a cache is written for a 2GB repository, the cache will go over the limit and caching will fail.