This issue occurs mostly when there is something wrong with FTP settings or the way server is defined.
Could you please make sure there is no " \ " prefixed or post-fixed in the server path.
Also which version of Lightroom are you using?
There are no " \ " in the FTP settings and I should add this hasn't changed. It worked OK then the reporting error occurred with no changes in the settings having been made. I am running Lightroom CC. I am currently exporting the gallery to my desktop and use a 3rd party FTP utility to load to my FTP site but would like to be able to load right from Lightroom.
I'm having the same issue with Lightroom CC2015.6. I noticed that the more images that I added to the Gallery, the more it started acting up. Finally now, I'm up to 200 images and it continues to give me the error no matter what. Very frustrating. I guess I'll have to Export and upload manually. Sure would be nice to do it from LR. None of my FTP settings have changed.