Microsoft's native antivirus software, Windows Defender, updated its definition list in late July, and is now being triggered by a non-malicious Jaxx system file. We have not had reports of this issue occurring with any other antivirus software. We are working with Microsoft to determine why the file has been flagged.
The simplest solution at this time is to open up Defender and list All Detected Items, select View details, locate "obfuscator.ui" and select Allow. You may also need to find the same item under the list of Quarantined items, then select Restore. This should grant you re-access to the Extension.
Alternatively, if you already have a secure copy of your wallet's backup phrase (and you should already!), you can de-activate Windows Defender and install an alternate antivirus program.