FYI. For those of us still running Win7 and MSE, it looks like Microsoft has turned off the automatic Update Definitions about 4 days ago: Searching the Internet, the only current reference to the issue was found in Digital Life. Uninstalling and reinstalling MSE did not fixed the issue. For now, the Security intelligence updates for Microsoft Defender Antivirus and other Microsoft antimalware page continues to provide current MSE definition versions, needing manual download and install, to keep MSE current. How long that will last is unknown.
From SevenForums... https://www.sevenforums.com/system-security/429985-microsoft-security-essentials-acting-strange.html