AD FS Event ID 383 , 199, web.config file is malformed

AD FS Proxy stopped working with Event ID 383

User Action:
Fix the malformed data in the web.config file.

or with you are found Event ID 199

Reason behind this is problem in config file microsoft.identityServer.proxyservice.exe.config located at

C:\Windows\ADFS\Config

You should take backups (and test those)
Best Practice

Normal file looks similar to below

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="microsoft.identityServer.proxyservice" type="Microsoft.IdentityServer.Management.Proxy.Configuration.ProxyConfiguration, Microsoft.IdentityServer.Management.Proxy, Version=10.0.0.0, Culture=neutral, PublicKeyToken=DDDD00999SSS, processorArchitecture=MSIL" />
  </configSections>

  <microsoft.identityServer.proxyservice>
    <congestionControl latencyThresholdInMSec="8000" minCongestionWindowSize="64"
      enabled="true" connectionTimeoutInSec="120" />
    <connectionPool connectionPoolSize="200" scavengeInterval="5" />
    <diagnostics eventLogLevel="15" />
    <host tlsClientPort="49443" httpPort="80" httpsPort="443" name="federation.affan.info" />
    <proxy address="" />
    <trust thumbprint="AAAAAAAAAAA00000AAAAA0000A0AA00AA0A0"
      proxyTrustRenewPeriod="21600" />
    <farmBehavior farmBehavior="2" />
    <ignoreBinding ignoreBinding="false" />
    <farmBehaviorLevel farmBehaviorLevel="4" />
  </microsoft.identityServer.proxyservice>
  <!-- <system.serviceModel>
    <diagnostics>
      <messageLogging logEntireMessage="true"
              logMessagesAtServiceLevel="true"
              logMessagesAtTransportLevel="true">
      </messageLogging>
    </diagnostics>
  </system.serviceModel> -->
</configuration>

Quick way is to replace the file if you have it backed up and service restart

If the solution above does not work remove and re-add the AD FS Proxy role and re establish the trust between AD FS and Proxy , This will regenerate the file and configs. From Administrative powershell run

Remove-WindowsFeature -Name Web-application-proxy
## Above command will require a system reboot after reboot run 

Install-WindowsFeature Web-Application-Proxy -IncludeManagementTools

Once the proxy feature is installed create the trust using following command

##Local Admin Credentials on AD FS Server 
$cred = Get-Credential

Install-WebApplicationProxy -FederationServiceName "federation.affan.info" -FederationServiceTrustCredential $cred -CertificateThumbprint "<Certificate Thumbprint>"

Done.