JabRef 5.3--2021-04-12--e3094ac
Linux 5.10.0-5-amd64 amd64
Java 15.0.2
JavaFX 16+8

I use a university proxy to access paywalled scientific journals.
When I try to use it within JabRef, it doesn't authenticate.

Steps to reproduce:

1. Set proxy server, port, username and password
2. Try to download fulltext
3. Download Failed

java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.0 407 Proxy Authentication Required"
at java.base/sun.net.www.protocol.http.HttpURLConnection.doTunneling(Unknown Source)
at java.base/sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(Unknown Source)
at java.base/sun.net.www.protocol.https.HttpsURLConnectionImpl.connect(Unknown Source)
at org.jabref@5.3.262/org.jabref.logic.net.URLDownload.openConnection(Unknown Source)
at org.jabref@5.3.262/org.jabref.logic.net.URLDownload.asInputStream(Unknown Source)
at org.jabref@5.3.262/org.jabref.gui.externalfiles.FileDownloadTask.call(Unknown Source)
at org.jabref@5.3.262/org.jabref.gui.externalfiles.FileDownloadTask.call(Unknown Source)
at org.jabref@5.3.262/org.jabref.gui.util.BackgroundTask$3.call(Unknown Source)
at org.jabref@5.3.262/org.jabref.gui.util.DefaultTaskExecutor$1.call(Unknown Source)
at org.jabref.merged.module@5.3.262/javafx.concurrent.Task$TaskCallable.call(Unknown Source)
at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)