An issue among issues. I'm able to download the new patch 5.0.3 , but it will not install. All it does is sit there as if trying to figure out what to do next. On my end, I've uninstalled and reinstalled the game, still the same issue. And I seriously doubt it's a Direct X or .NET issue. It would be nice to be able to play again. Maybe I'll go play something else not written in .lua while I wait once again for the issue to fix itself.........
Well, it seemed to me that when patching the game late this morning (Tuesday, after maintenance) that it was taking rather long for the autopatcher to actually start downloading the patch, prior to unpacking and installation. It might not have had its coffee and was sluggish. I don't know. But it did finally get around to downloading. (Today's patch was 200MB so your download speed will almost certainly vary.)
After updating the manual patch list and testing the download--which works, your're welcome--I was able to download the patch before the client was finished downloading and installing it. Go figure.
Give it another try and just be patient, it could take awhile.