Solved TDAgg3.dll was not found

A similar thread could be found here and indeed adding the game directory to exceptions list will solve the false positive error as it's also mentioned on our website.
I think microsoft made a new update recently and windows defender throws a false positive alert of the library, although I didn't experienced it yet. This isn't the first time one of the microsoft updates breaks something as reported previously in here when even generic applications like google chrome were affected and they're still fixing problems caused by that update, hopefully it won't be the case this time too.

If you are using Windows Defender then one of their recent updates will trigger a false positive on various other applications including this library file.
FIX: Add the complete diablo game folder to exclusions lists as shown in this video and restore or re-download the TDAgg3.dll file from here or re-apply server patch.

UPDATE 1: God damn it microsoft! Indeed, this is a false positive caused by the latest windows defender definitions update and apparently lots of other legit applications are experiencing the same problem now, luckily most people don't use this lame defender and the update seems it wasn't pushed for everyone either. For the moment make sure you add the game folder to exclusions if you have any issues.
We already submitted the required files so the false positive alert is removed, hopefully it won't take long to be verified and whitelisted.

UPDATE 2: Alright, seems that microsoft already confirmed this was a false positive error on their end as you can see here, therefore it should be removed and whitelisted shortly in the up coming updated definitions of their software. In order to update your microsoft windows defender definitions you can also use this guide https://www.microsoft.com/en-us/wdsi/definitions

UPDATE 3: Microsoft removed the false positive for older operating systems as well. Please try the following steps to clear cached detections and obtain the latest malware definitions:
1. Open command prompt as administrator and change directory to c:\Program Files\Windows Defender
2. Run “MpCmdRun.exe -removedefinitions -dynamicsignatures” The latest definition is available for download here: https://www.microsoft.com/en-us/wdsi/definitions
Best regards, Windows Defender Response

Confirmed, microsoft has corrected the false positive.