In case you don't want to open Reddit, here is a step by step of how the workaround goes
-Upload your Realm
-When prompted about the 504 error, wait around 1-5 minutes
-Go to your world back ups on the Realm settings and you should have a 1-5 minutes old backup created. Restore it
This worked for me on 4 different occasions
Possible fix found by reddit user Bradyevander098
https://redd.it/1ki7a60
I tried it myself and got it to work
it is amazing how
-Slow upload speeds
-Slow processing rates
-Unresponsive/no explanation from the bug itself
and for almost 4 weeks now and ZERO feedback from Mojang
week #3, no solutions yet
Issue hasnt been addressed. Error 504 and 401 still happening
proposed fix doesnt work
Can report that issue is still happening
Getting the same
I get either 504 or 401. Been happening for the past 3 days
despite the workaround, this isnt solved yet