I'm assuming a .docx extension, since you missed that bit out. Doesn't matter what it is as long as there's something!
One problem: Your regex "find everything after the last backslash and replace it with itself then two underscores then itself again". So your prompt list text for the string above is actually
/Users/ronald/Documents/Dropbox/Jerry IMAC backup of important systems files/Web Site Registrations and Online Access/Web Site Registrations and Accessibility by Jerry CURRENT FILE filtered by accessibility and reliability10 Aug 2022 at 19-57.doc__Web Sites__Web Site Registrations and Accessibility by Jerry CURRENT FILE filtered by accessibility and reliability10 Aug 2022 at 19-57.doc__Web Sites
...with __Web Sites at beginning and end of the string the prompt displays. That's just a display issue though. The resulting file path is:
/Users/ronald/Documents/Dropbox/Jerry IMAC backup of important systems files/Web Site Registrations and Online Access/Web Site Registrations and Accessibility by Jerry CURRENT FILE filtered by accessibility and reliability10 Aug 2022 at 19-57.doc
...which matches your input path.
But... Dropbox. Are all the files on Dropbox also stored locally, or are they online-only? I've seen problems with other apps, eg backup software, when trying to access Dropbox's online-only "stub" files.
Because when I make a new Word doc with the following path, even longer than yours, the macro works fine:
/Users/nige_s/Dropbox (organisation removed)/Jerry IMAC backup of important systems files/Web Site Registrations and Online Access/Web Site Registrations and Accessibility by Jerry CURRENT FILE filtered by accessibility and reliability10 Aug 2022 at 19-57.docx
...so there's no issue with path length.