Unfortunately in this case it is a discontinued product version so is not eligible for ongoing support. Our team often does their best to provide guidance in these cases but are unable to go into the issue very deeply.
That said, the "failed to connect to database" message in a general sense means the internal camera ID behind the friendly name you see in the Smart Client is unrecognized by the server.
This can happen when a camera has been removed and readded or removed and replaced without updating the view in Smart Client for example.
It could also be something more nefarious like the media files being locked by antivirus or backup software for example.
I would verify and try the following...
0. Open hardware properties, select the IP/license tab and re-enter the username and password, followed by clicking OK. Clicking ok will trigger a reauth with the camera similar to when the camera is added. You can get a failure here if the credentials or selected driver are incorrect, or if two cameras in the configuration have somehow swapped IP addresses (common with DHCP).
1. Check the license status. Are they in 30 day grace? Has the grace expired?
2. Check user rights and verify the Smart Client user has the necessary permission to at least view live for example.
3. Save any changes and restart the Recording Server service
4. Right click on the Recording Server icon and show status. Are all cameras green? Some grey? Red?
5. In the same icon, show the recording server log file. Any obvious errors? Some common ones are "cannot retrieve serial number" or "camera failed to provide an image". These can indicate authentication trouble, incorrect IP or MAC of the camera, or Windows Firewall blocking a stream. (Management Application might be allowed to get the stream and not the recording server, hence the different experience between smart client and management application)
6. Create a new view in Smart Client while in setup mode. Then drag and drop the cameras in to see if the issue persists.
Those steps should clear up 90% of issues. In extreme cases you might have a corrupt live database table or filesystem. You can try a chkdsk /F on the Recording drive, optionally followed by deleting the live folder for those cameras. This isn't ideal obviously, and almost never necessary but without seeing the issue first hand I thought I'd give you all the typical ideas we'd pursue.