This is a widely reported problem with Outlook after the January Update, that update is affecting Outlook POP accounts, this is being reported by many people after that update, some people have reported if you uninstall the Windows update, that will resolve the problem, also, it is best to open the Feedback Hub app in Windows and report this to the Windows developers, if more people report this, then they will investigate the problem and provide a patch for this problem.
Microsoft provided a patched update for other reported problems in that update, but have not yet released a fix for the Outlook bug, everyone is waiting on a fix for this.