Solr fails to start on Windows with obscure errors when using relative path

Clean 5.3 (and 5.3.1 RC3) on Windows:

• bin\solr start -e techproducts
• Visit Admin UI - all works
• bin\solr stop -all
• bin\solr start -s example\techproducts\solr
• ERROR: Solr at http://localhost:8983/solr did not come online within 30 seconds!
• Visit Admin UI - get an error:

  HTTP ERROR 500

  Problem accessing /solr/. Reason:

  Server Error
  Caused by:

  java.lang.NullPointerException
  at org.apache.solr.servlet.SolrDispatchFilter.authenticateRequest(SolrDispatchFilter.java:237)
  at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:186)
  at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:179)
  at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
  at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
  at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
  at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
  at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
  at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
  at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
  at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
  at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
  at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
  at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:215)
  at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:110)
  at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
  at org.eclipse.jetty.server.Server.handle(Server.java:499)
  at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:310)
  at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)
  at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:540)
  at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
  at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
  at java.lang.Thread.run(Thread.java:745)

Possibly related to SOLR-8068?