P: Desktop: Post error (504) when uploading .raf-files

I uploaded photos from an event yesterday, .nef and .raf raw files, and after the lightroom cc desktop clients syncing was complete, I checked the iOS version and the browser-based version, where I only saw 'error tiles' on the .raf-files (see screenshot 1 and screenshot 2). I then did a reset of the settings on the desktop app (by renaming the settings file) and deleted the .raf files and re-uploaded them, which didn't work. 

I remembered that i had this error a few weeks before, so i once again deleted a few .raf files and tried to upload one over the browser-based app, which resulted in a post-error with a http-response status code of '504 Gateway Timeout', as seen in screenshot 3. Back then, I thought this has to be a server error (and not client-side) so i let it rest for a few days and it did resolve by its own. But now it happened again. 

Notice that this only happens (at least for me) with .raf files, the .nef files are displayed and synced perfectly. In the desktop app, the raf files are also displayed normal and their sync status for 'Cloud' is 'Original — RAF'. Also, no 'Sync Errors' are displayed, so maybe it's just a rendering issue? Nevertheless, help is much appreciated.