This article is for you if you have a Dahua DVR, NVR, and got an "invalid username or password" error. Here I explain what can cause such error and how to fix it.

Be aware that this error also occurs with Dahua OEM products.
Here's a list of devices that this article applies to.
Activecam, Advidia, Amcrest, Ameta, Ascendent, Backstreet Surveillance, BV Security, CCTV Security Pros, CCTV Star, CP Plus (Orange Line), Dahua, Dax Networks, eLine, ENS (formerly Eastern CCTV and SavvyTech), Expose, Lorex, GSS, Honeywell, IC Realtime, Ikegami, Impath Networks, Inaxsys, IndigoVision, Infinity CCTV, Innekt, Intelbras, KBVision, Lumixen, Maxron, Montavue, Oco, Optiview, Rhodium, RVI, Saxco, Security Camera King (Elite), Space Technology, Speco, ToughDog, Tyco Holis, Tyco Illustra Essentials, Unisight, VIP Vision, Watchnet, Winic, Zuum.
What causes the invalid username or password error
There are different causes for the invalid username or password error; most of the time, it's related to the credentials' misconfiguration in the NVR or camera.
The first thing you need to do is to keep calm and don't get stressed.

When you set up the NVR for the first time, the credentials are shared with the IP cameras connected to the network, that's the usual approach for a fresh installation because it helps you have everything working quickly.
Suppose you reset your NVR and change the credentials to recover from a lost password. In that case, the new password you use is not updated in the camera, and that causes a login error when the NVR tries to send the username/password combination to the cameras.
The other common reason for the invalid username or password error is the use of the wrong credential due to incorrect typing. Keep in mind that the camera and NVR are aware of case sensitive letters and spaces you could be using while typing.
How to fix the invalid username or password error
The best way to fix the invalid password or username error is to double-check if the NVR and IP cameras use the correct credentials.
You can check which cameras have the wrong credentials by opening the camera registration menu in the NVR and checking the camera status.
The IP cameras with problems show up with a red dot, as shown in the picture.

To fix the problem, click on the "Modify" pencil to change the camera credentials.
A new screen pops up to allow you to change the camera's password.

Change the password and click the "Connect" and "OK" button to finish.
That changes the password for the camera and fix the error.
Other problems
There is a limitation in the number of attempts to use a password.
This limitation is meant to protect your device from people trying to log in with different username and password combinations.
If the camera gets 3 login attempts in a short period of time it locks itself for some minutes and doesn't allow any new login from the NVR.
You can restart the camera to attempt a new login and if the problem happens again just isolate the camera from the network and change it's password manually.
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Conclusion
The invalid username or password error shows up on Dahua and other devices manufactured by this company as OEM.
This article brings the solution for this problem but you need to consider try different approaches by yourself as long as you understand the concept.
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