Replying to my own message in case it's of use to someone else:
It looks like this can be fixed by commenting out the 'target:
"__use_router__"' lines in client/galaxy/scripts/layout/menu.js i.e.
diff --git a/client/galaxy/scripts/layout/menu.js
b/client/galaxy/scripts/layout/menu.js
index
Hello Ping (and Galaxy-devs)
Did this get resolved? We are still on 18.09 and serving our production
instance with a proxy prefix, and one of my users has hit this issue when
trying to access the "Workflows" tab from the main page.
As Ping says, it appears that Galaxy is inserting an extra
