add note about deleting .csync_journal.db for "CSync unknown error"

doc/troubleshooting.rst

@@ -51,6 +51,11 @@ Identifying Basic Functionality Problems
   ``propget`` command to obtain various properties pertaining to the current
   directory and also verify WebDAV server connection.
   
+"CSync unknown error"
+---------------------
+
+If you see this error message stop your client, delete the ``.csync_journal.db`` file, and then restart your client. There is a  ``.csync_journal.db`` file inside the folder of every account configured on your client. See https://github.com/owncloud/client/issues/5226 for more discussion of this issue.
+
 
 Isolating other issues
 ----------------------