Error 500 when purge cache in nginx helper

using v1.9.8, when click purge (all) cache i get an error 500 + message saying my site is not working.

redis is up and running.

anybody else has this error? the site seems to be working just fine, and serving pages from cache correctly…except that it doesnt seem to show some of the changes i have done in css. some pages are updated, others are not.

really weird.