Discovery Scan Issue - Linux scans returns NoValidTargets error

Hi mates, 

I am in a trouble with Discovery Scan and each time it can surprise me with new issue.

I can not scan Linux  and Linux based assets (like Palo Alto firewalls) with Discovery Scan. It  displays the error “NoValidTargets” error altought is was working properly yesterday and suddenly started to give this error.  There is not any 

• Network connections issue. Appliance can connect to SSH ports of Linux machines.
• Credentials issue. I use root user and tested its password a few times, no fail while manually login.
• No service error. I checked with getc, teste, testc commands.
• I tried on new VMs and formerly scanned ones. All scans returned same error mentioned above.

Everything is ok with Windows scans.