Have you tried "View Source" for clues what the rule might be?

There are numerous HTML rule instances. 

In Records view, click on Technical >> HTML.

In the Stream Name column you can search for text such as "Auth", for example, .

Or just do an all-out content search for somewhat-unique text string.

HTML rules should be Available for modification, i.e., can be overridden,

Hi

as pedel suggested take a look at the Live UI to find the html or the fragment. In case you are referring to the ActivityStatusError screen then you can edit the HTML amd remove the function which concatenates the build information.(pxGetBuildInfoString)

Moreover if this is related to external authentication , you can also show a custom html in case of failed login attempts. Check the Custom tab in the authentication service rule for different options to handle failed login attempts.

Thanks

Check below link, it explains how to customize login page

https://community.pega.com/knowledgebase/articles/customizing-pega-7-login-screen