Windows error: The client and server cannot communicate, because they do not possess a common algorithm

You may come across this error if you've disabled TLS 1.0 on your system as the agent will no longer be able to post payloads back to Server Density. 

You can work around this issue on fully updated servers by adding specific registry values. These can be added manually via regedit, or by using the commands provided.

Update

First, make sure that your Windows server is up to date, including all updates for .net3.5.

Required Registry Values

In order to use the OS defaults SSL and TLS instead of the hardcoded .NET Framework defaults you need to create a new SystemDefaultTlsVersions DWORD with the value of 1 at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727.

64Bit Servers will also require the same DWORD with the value of 1 at  HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v2.0.50727.

Using regedit

Open regedit.exe as an Administrator. 

Once open navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727 and then right-click and select 'New' > 'DWORD (32bit) Value'.

Rename the new value to SystemDefaultTlsVersions.

Modify the new value and set the 'Value Data' to 1. 

For 64 Bit servers repeat this process for HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v2.0.50727

creating another new value with the same SystemDefaultTlsVersions name and Value Data of 1.

Using cmd

You can also set these registry values quickly and easily via the command line. Open cmd as an administrator and execute the following: 

reg add HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v2.0.50727 /v SystemDefaultTlsVersions /t REG_DWORD /d 1

For 64bit servers, add the additional required value:

reg add HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v2.0.50727 /v SystemDefaultTlsVersions /t REG_DWORD /d 1

Reboot

In order for these new values to take effect, you will need to reboot the server. Once rebooted you should see metrics for the server in your Server Density account.

Further Reading

The above information was taken from the following Microsoft document which you can refer to for further details: 

https://support.microsoft.com/en-us/topic/support-for-tls-system-default-versions-included-in-the-net-framework-3-5-on-windows-8-1-and-windows-server-2012-r2-499ff5ef-a88a-128b-c639-ed038b7d2d5f