Blizzard has explained Diablo 2's server issues and says a fix will take some time to roll out.
Today Blizzard published a lengthy blog post detailing what's going on with Diablo 2's servers. Apparently D2R uses the same server structure as the original game, with minor alterations and tweaks--these tweaks are becoming more plentiful thanks to these issues. The devs explain that the server issues the past four days were caused by a perfect storm of massive concurrent user spikes and a game-improving update. Both of these combined knocked the servers offline. Datacenters have simply been overwhelmed every day--too many people are trying to play Diablo 2 Resurrected at once.
The global servers were also trying to continually reconnect and back themselves up, an automated process that caused lots of headaches. So Blizzard has optimized and released fixes with server downtime. The devs also say that the biggest fix will take some time to roll out.
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Diablo 2 Resurrected has two different types of servers: Global and regional. Regional servers will connect to global servers and upload character and account data. The issue is these uploads happen far too often. The original D2 had a lot less players and this didn't get in the way very much back then.
Blizzard is now coding a solution that will significantly reduce how often regional servers upload to global servers. This will free up the servers to back up data to their own regional databases.
This fix will take some time, though:
"Right now we are writing code to change how we do this entirely, so we will almost never be saving to the global database, which will significantly reduce the load on that server, but that is an architecture redesign which will take some time to build, test, then implement."
Long story short: You should probably expect intermittent server downtime and core issues throughout the coming weeks.