I started receiving this error the other day:
After doing some digging, I found out that the LEM pulls/synchronizes connector updates from http://146.112.239.215:80 (downloads.solarwinds.com). I put the address in my web browser and I received an umbrella notification saying that the page was being blocked.
After whitelisting the address in Umbrella, the connectors started downloading successfully again and I haven't had issue since.
I hope this helps!