I am using Joomla 3.7.3 together with gantry 5.4.13 (template: Paradise by InspireTheme) in a multi-language setup. With securitycheck 2.8.19 everything works fine, however when updating to 2.8.20 it breaks my site, i.e. my default language (English) is still working but when switching to other languages I get 404 errors. For now I will stick with 2.8.19.
I have done many tests and I'm not able to reproduce this issue. Could I access to your site to check what's happening? If so please send me administrative credentials to the email from which you receive forum notifications.
P.S: ¿Eres español? Si es así podemos hablar en español si te es más cómodo (yo soy español )