I am using an Alienware m15 gaming laptop with 32GB of RAM running Windows 10 Home 64-bit. I have also run into the same issue running Minecraft on my Xbox Series X. I have a fiber Internet connection from Hargray Communications. I am connected via Wifi with download speed currently 341.77Mbps down and 58.77Mbps up and 6ms Ping. My NetGear Nighthawk router reports wired speed of 782.05Mbps down and 62.23Mbps up and 4ms Ping.
 
I purchased the Minecraft Bedrock edition from the Microsoft store and it is up to date running the current version 1.19.22. I have a 10 Friend Realms subscription that I started from the Xbox Store. 
 
When I download my world to my laptop it is 176.6MB in size. I have 1 Resource Pack active, which is Global Resources. I have 1 Behavior Pack active, which is Default. I can play the game locally using the world downloaded from Realms. 
 
When I try to go into the settings for the local copy and select Play on Realm, select my existing realm and choose the slot titled "My World" (or any of the other 2 slots), confirm that I want to replace my current Realm world with my downloaded copy, it begins the export process, then says it was exported successfully. The initializing upload screen loads but the thermometer bar never shows any indication of uploading. After approximately 60 seconds the "Error" box appears saying "An error occurred" with the only option "OK". It has changed my active slot to the "My World" game but has not replaced the world with my locally saved version.
 
I can change my active slot back to "Frozen world" and the game loads the Realm that is already in that slot.