My impression is that it is the RESORB download that is not working in GPT. When I process a product from before 20 (+ a few) days ago, it gets the POEORB files and succeeds.
Downloading the files with PyroSAR and having SNAP pick them up works for me, after some confusion because file system permission issues give exactly the same error message as files not being available online.
PyroSAR has a similar logic to SNAP, cycling through different endpoints and trying to get the different products:
wrapped into a function here
Very descriptively, they call their gathering the orbit files to
catch them - they seem to be really hard to get a hold of these days.