=== modified file 'Mailman/Cgi/options.py'
--- old/Mailman/Cgi/options.py	2021-10-18 23:56:42 +0000
+++ new/Mailman/Cgi/options.py	2021-11-03 19:02:21 +0000
@@ -346,6 +346,8 @@
             varhelp = qs[0]
     if varhelp:
         # Sanitize the topic name.
+        while '%' in varhelp:
+            varhelp = urllib.unquote_plus(varhelp)
         varhelp = re.sub('<.*', '', varhelp)
         topic_details(mlist, doc, user, cpuser, userlang, varhelp)
         return