Not can, will. CASC craps all over itself if you play while it’s downloading data, especially if you’re playing in an area that requires said data being downloaded. The CASC filesystem does not like having a download being accessed before it is fully written with proper permissions (the OS will not allow certain permissions on live files). This applies to both Windows and macOS.
I really wish Blizzard would just take away the option to even click play before the installing/patching is completed. It’d save a metric crapton of headaches for players down the road.
Another fun thing happened today after waking up the PC from sleep and starting D3 for some more runs. The game started without any extra downloading and in game I could press potion (just to test) but no potion power ring showed up and in porting to act 1 no altar was present 
. An exit and restart from launcher made things right again (and without any downloading showing).
This is a bug that only seems to affect the EU region.
It happened to me once. I start playing and after a few minutes, I think, “Okay Strix, something’s missing.”
No Altar or seasonal theme ^^
I just left the game (not D3, just the current game) and restarted. And everything was restored.
Now I go straight to Act 1 to check if the altar is there.
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Yes. Many times. Launcher didn’t found anything wrong
Then you need to remove the game files and re-download them as I mentioned here:
-"Downloading Additional Content" - #79 by DTMAce-1687
Not helped. Launcher wasn’t running. I 've deleted all contents in data folder. Run the launcher. Play button was active. I’ve pressed to scan and repair download many data. Not launch the game until all download. But after PC reboot there is again the message.
I’ve moved game to SSD and it solved this problem.
I’ll try and move D3 to another drive and see if it works (EU player like you… and EU seems to be the problematic region). There are youtube videos on how to move battlenet games…
Got an SSD drive for D4 a year or two ago 
Just copy /Diablo III to SSD and select new path in Battle.net app.
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Usually the button will say something like install or update. Not play after deleting the data folder.
Again. Make sure the launcher is closed before moving the game files. But if moving the game files fixes the issue, then there may be a problem with the original drive location. As in the drive having issues, causing the game to fail.
i dont think this issue “Downloading Additional Content” is player side. It’s certainly server side and something they dont think we should know about. i’d be willing to bet my reputation on it.
And yet, examples posted in this thread indicate otherwise.
I have never had this message, in all the years since the game launched that I can even recall at this point. 100% sure of not seeing it in recent years.
It could be file database related, or a mismatch between your local copy of files and the server due to your campaign status.
Believe it or not, there are players that have never completed the campaign on their account. I have seen that just by visiting player profiles. That also could be a factor as noted with a few other posts in this thread:
well, the fact that nobody can pinpoint the cause at all, and the fact that some people get it and some dont, and the fact that myself have it occur on 6 different pc’s new and old all separate configurations, tell me it’s server side and that it’s likely location based on local privacy laws.
Maybe the game doesn’t like you.
Or you are running something most of us aren’t that’s causing it. Who knows.
you ever wonder why blizzard doesnt acknowledge it, especially if it’s something simple…
Nope. I don’t wonder about it at all.
Honestly don’t really care that much either. But that may be due to not having the issue myself.
*shrug
Maybe if every single player that was having this problem all complained at once, that might make them take notice?
I think I’ll just reinstall the whole thing if scans don’t work. Keeps crashing or disconnecting entirely at the most random times. I have checked processes that work at the background and apparently those are not the issue. I have deleted a few suspiciously corrupt file here and there on the disk, thinking that these could cause it but crashes didn’t stop. Guess the only option is uninstall and then a fresh reinstall for me soon.
He also reported that the next day it was back to “downloading additional content”. So it wasn’t fixed. You only get the extra downloading the first time you play the game after a PC restart/ reboot. Quitting and starting more times that day doesn’t require any more downloading.
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i dont want to say dont bother but it doesnt work for anyone else.
another reason why i believe it’s server side.