That specific problem does not like it is n8n or Dropbox related. More like an underlying issue with the network/connection. Did you try Google the issue?
Does only that node fail or also others? Are you sending a lot of data or making a lot of requests? Or do you try to upload just one small file?
Because if you would send a large file (or also many small ones, the combined amount matters), especially with the editor-UI open (as that doubles required data usege) the chance is quite high that it becomes unresponsive because n8n crashes. That would also explain why the browser becomes unresponsive as all the data does also get sent to the browser and if it is to large it would also crash/freeze.