Wrong files delivered from plone through varnish

we are running 4 plone instances behind a varnish. last week the varnish delivered different files depending if accessed directly or through a non-caching apache mod_proxy. Directly connecting to varnish (:8100) .../foo.js and .../foo.css were delivered correctly as two different files. Connecting through the apache proxy (RewriteRule with Virtualhostbase and VirtualHostRoot to :8100) we got the same file (the css) for both URLs. there is abolutely no caching on the Apache side only Rewriterule.
Restarting varnish and the zope instances (and recompiling ressources from the resource registry) solved the problem.
Any idea how to search for a reason for this strange behaviour? None of the 4 instances showed this problem when accessed directly. Could this be more a varnish or a plone problem (or a configuration and therefore administrator problem)?
Our varnish vcl can be seen here: https://pastebin.com/jmJN1Q3H