I don’t know what was the previous version, but we update the plugins regulary.
Could it be because of the fact that we have the WordPress in Portuguese language?
The theme we are using is Townpress but in the frontoffice, the seacrh works well, so it is not a matter of te theme.
The WordPress version is 4.9.7 (it was automatyicly updated a few days ago)
Is the WordPress search works well with you, the only diference that I see is the language… could this be the problem?
If we put this in the URL (without categories and tags) and hits Enter, it works well: