Hi,
The other search form works, because it only has post type search active, it only searches “projects”, and the results page can handle that. If you were to enable members search (user search) on that form, it would very likely trigger the same error message.
There was a minor misconfiguration issue with the members search, the custom URL was not set correctly (the query variable), I logged in and corrected that. After hitting the return button I am now redirected to the other page where the search is triggered and the results are displayed now.