New OpenRefine install produces almost blank landing page (no menus) - openrefine

Question

OS: Windows 10 Pro 64 bit 19042.985
JDK: OpenJDK 16.0.1+9-24
OpenRefine: openrefine-3.4.1
Browser: Firefox 88.0.1
Just installed OpenRefine for the first time, on top of a fresh install of OpenJDK 16
The server runs and opens the URL (localhost:3333) in the browser, but the page that comes up is almost blank. There is the OpenRefine logo in the top left corner, but the rest of the page is empty. There is no UI (menus).
--UPDATE--
Looking at the page html in more detail it's not strictly true that there is no content in the menus. There are a small number of links in [li] tags, but these are being rendered as 0-width elements by the browser (in either Firefox or Edge) so they are no visible on the page.
By the looks of it the page is failing to load several external references including the stylesheets, and I suspect that's linked to the bad rendering of the page.
--END UPDATE--
The OpenRefine log output is-
12:39:39.088 [ refine_server] Starting Server bound to '127.0.0.1:3333' (0ms)
12:39:39.088 [ refine_server] Initializing context: '/' from 'C:\OpenRefine\openrefine-3.4.1\webapp' (0ms)
12:39:39.458 [ refine_server] Failed to use jdatapath to detect user data path: resorting to environment variables (370ms)
12:39:39.458 [ refine_server] Failed to use jdatapath to detect user data path: resorting to environment variables (0ms)
SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/C:/OpenRefine/openrefine-3.4.1/server/target/lib/slf4j-log4j12-1.7.18.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/C:/OpenRefine/openrefine-3.4.1/webapp/WEB-INF/lib/slf4j-log4j12-1.7.18.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.slf4j.impl.Log4jLoggerFactory]
12:39:39.573 [ refine] Starting OpenRefine 3.4.1 [437dc4d]... (115ms)
12:39:39.573 [ refine] initializing FileProjectManager with dir (0ms)
12:39:39.573 [ refine] C:\Users\coev\AppData\Roaming\OpenRefine (0ms)
12:39:39.573 [ FileProjectManager] Failed to load workspace from any attempted alternatives. (0ms)
12:39:43.367 [ org.mortbay.log] /images/favicon.png (3794ms)
java.lang.IllegalStateException: Committed
at org.mortbay.jetty.Response.resetBuffer(Response.java:1024)
at javax.servlet.ServletResponseWrapper.resetBuffer(ServletResponseWrapper.java:202)
at org.mortbay.servlet.GzipFilter$GZIPResponseWrapper.resetBuffer(GzipFilter.java:310)
at org.mortbay.servlet.GzipFilter$GZIPResponseWrapper.sendError(GzipFilter.java:319)
at edu.mit.simile.butterfly.Butterfly.error(Butterfly.java:1020)
at edu.mit.simile.butterfly.Butterfly.service(Butterfly.java:528)
at com.google.refine.RefineServlet.service(RefineServlet.java:210)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserAgentFilter.java:81)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter.java:132)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:547)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.jetty.bio.SocketConnector$Connection.run(SocketConnector.java:228)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
at java.base/java.lang.Thread.run(Thread.java:831)
I originally installed OpenRefine in C:\Program Files\OpenRefine\openrefine-3.4.1 but thinking this might one of those Unix-native apps that can't handle pathnames with spaces this log is from a second install in C:\OpenRefine\openrefine-3.4.1.
When OpenRefine runs, with the first install (in Program Files) just a blank workspace was created in C:\Users\coev\AppData\Roaming\OpenRefine. With the second install directly in C: it gets as far as creating a workspace.json file in that folder, so that's one difference.
Looking at the html page source generated by OpenRefine the html looks valid, but the blocks for the left and right menu panels are empty.
I have searched but haven't found this specific issue decribed elsewhere. Any ideas?