I'm also having issues while syncing with Joplin Cloud:
Completed: 15/09/2021 10:04 (265s)
Last error: Error: Unknown error: <html> <head><title>504 Gateway Time-out</title></head> <body> <center><h1>504 Gateway Time-out</h1></center> <hr><center>nginx/1.18.0 (Ubuntu)</center> </body> </html>