I've heard reports from several people today that have identified issues where the WSUS logs are giving error messages and the server is failing to synchronize updates.
Microsoft already has a KB article assigned to the problem, providing a workaround. The issue affects Windows Server Update Services 3.0 Service Pack 2 and Windows Server Update Services for Windows Server 2012.
Microsoft is still working to figure out the problem, but has given a temporary fix to ensure you can synchronize the April updates. The fix involves disabling sync, running a manual sync, reenabling sync, and performing a sync once again. Additionally, these steps have to be performed for each specific WSUS category.
The workaround is supplied in detail here: WSUS silently fails to synchronize updates released in April 2015