Error 500 thrown by Solr server on admin/reports/status

I successfully added a Solr server to admin/config/search/search-api which is up and running. No error messages on its status page so far. "The Solr server could be reached." and "The Solr core could be accessed (latency: 48.8450990448 ms)." and "Configured Solr Version 7.3.0".

I added an Index to it, which is also running.

The problem I have: if I call admin/reports/status, I get a white screen saying "The website encountered an unexpected error. Please try again later." which corresponds with a php error: Drupal\search_api_solr\SearchApiSolrException: Solr endpoint http://my.solr.server:8993/ internal Solr server error (500). { "responseHeader":{ "status":500, "QTime":2}, "error":{ "trace":"java.lang.NullPointerException\n\tat

The Solr server log reports:

java.lang.NullPointerException
	at org.apache.solr.handler.admin.ShowFileRequestHandler.getAdminFileFromFileSystem(ShowFileRequestHandler.java:329)
	at org.apache.solr.handler.admin.ShowFileRequestHandler.showFromFileSystem(ShowFileRequestHandler.java:190)
	at org.apache.solr.handler.admin.ShowFileRequestHandler.handleRequestBody(ShowFileRequestHandler.java:134)
	at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:195)
	at org.apache.solr.core.SolrCore.execute(SolrCore.java:2503)
	at org.apache.solr.servlet.HttpSolrCall.execute(HttpSolrCall.java:711)
	at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:517)
	at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:384)
	at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:330)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1629)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:533)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
	at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:548)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:190)
	at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1595)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:188)
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1253)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:168)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:473)
	at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1564)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:166)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1155)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
	at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:219)
	at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:126)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
	at org.eclipse.jetty.rewrite.handler.RewriteHandler.handle(RewriteHandler.java:335)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
	at org.eclipse.jetty.server.Server.handle(Server.java:530)
	at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:347)
	at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:256)
	at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:279)
	at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:102)
	at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:124)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:247)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.produce(EatWhatYouKill.java:140)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:131)
	at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:382)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:708)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:626)
	at java.lang.Thread.run(Thread.java:748)

All I understand is that some file request has failed. But which file could that be? I moved all the files that I got in config.zip onto the Solr server. If I disable the Solr server on Drupal side, I can reach admin/reports/status without an error message.