Initial report: https://en.wikipedia.org/wiki/Wikipedia:VisualEditor/Feedback#Error_contacting_the_Parsoid/RESTBase_server_(HTTP_403)
E.g. try to create https://en.wikipedia.org/wiki/1.2.3.4?veaction=edit or https://en.wikipedia.org/wiki/97.72?veaction=edit
Not a problem if the page already exists, e.g. https://en.wikipedia.org/wiki/97.7_FM?veaction=edit
This seems related to T232556: Parsoid/PHP roundtrip testing: Period ( . ) in titles fail with a http 403 (Invalid file extension found in the path info or query string.) error. But now that IE6/IE7 support has been removed, it is possible this will unbreak once that code rides the train.