Do you have the Avast firewall on?

Was the windows firewall actually turned on before Avast installed? i.e. was it running as a standard setting in your domain.

If it was running it sounds like manual rules set in the firewall for allowing LANdesk have been whiped.
If you (for testing purpose) uninstall Avast without touching anything else, can you connect then?

I would advise to lookup the ports needed for LANdesk and put them in a grouppolicy setting as rules for the windows firewall OR if you have Avast firewall, in Avast.