Another 'weird' problem, however this seem to be related to the Joomla 3.9.0 version. Situation happens when we have menu item pointing to the SobiPro search, located under alias type menu item. From some reason in the latest Joomla version when we try to search we are redirected to the wrong url and in the result it ends with 404 error page.
Here is example installation:
http://test3.sikora.sobipro.eu/alias/search
One note, at some installations it seems to be related to the Gzip compression, but I already saw two sites when it happens also with Gzip disabled