I had recently had this error in WSUS where my Windows Server 2016 servers would NOT report into the WSUS Server. I would get an error stating 0x8024401c when manually performing a report now to the WSUS Server using:
Go to IIS Manager on the WSUS Server
Goto Advanced Settings of WsusPool.
Make sure following settings are present/configured on the Pool, if not change it to below:
Queue Length: 25000
Limit Interval (minutes): 15
"Service Unavailable" Response: TcpLevel
Private Memory Limit (KB): 0
Make sure, the WSUS Entry in the Registry is having fully qualified domain name of WSUS Server.
NOTE: If you have Group Policy managing the WSUS Settings, then make sure you change the settings in the WSUS Policy to use the FQDN of the WSUS Server and run a gpupdate /force on the clients.
Stop IIS on the WSUS Server
Edit the web.config located at following location on WSUS Server:
C:\Program Files\Update Services\WebServices\ClientWebService\web.config
Replace the following lines in the config file and save in the same directory:
"<httpRuntime maxRequestLength="4096" />"
"<httpRuntime maxRequestLength="204800" executionTimeout="7200"/>"
Restart IIS on the WSUS Server
Try updating the clients again. They should be able to report and update successfully.