The latest version of InDesign (v15.1) contains an issue in the World-Ready Composer that displays all variable text (e.g. page numbers, etc.) as "ss".
If you are using the World-Ready Composer, it is not advisable to update to this version of InDesign.
There are more details to be found on this InDesign Secrets page.
You can vote for this issue to be fixed at this InDesign UserVoice page.