I had opened a ticket concerning this issue and actually got an email back. The steps to follow (copied from said email) are:
>Close the SW:TOR launcher.
>Download the launcher repair file here : http://cdn-patch.swtor.com/patch/LauncherRepairUtilityD2020.0.6.exe
>When you’re asked, choose to Save the file.
>Copy or move the downloaded file to your SW:TOR game folder:
32-bit Windows - C:\Program Files\Electronic Arts\BioWare\Star Wars - The Old Republic\
64-bit Windows - C:\Program Files (x86)\Electronic Arts\BioWare\Star Wars - The Old Republic\
>Once you’ve copied the file, double-click on it to run it.
>Choose Install, and let it run.
>When it’s finished running, it opens the launcher. Try logging in and patching again.
However, I solved the issue for me by finally reinstalling the game and deactivating streaming mode. I suppose it doesn't matter if you uninstall the game or not, deactivating streaming mode is always followed by downloading the whole game (~40GB according to my game folder) ... however, after doing that everything worked fine.
Deactivating Streaming mode:
- Go to the SWTOR directory (usually C:\Program Files (x86)\Electronic Arts\BioWare\Star Wars - The Old Republic)
- Open the file "launcher.settings" using notepad or notpad++ (any texteditor will do)
- change "bitraider_disable": false to "bitraider_disable": true
- save the file
- start the launcher
Good luck, may the downloadrate be with you ... :-)