Check your firewall settings, [...]
Huh ?
A single dropped node has nothing to do with the firewall.. at least if this specific IP isn't explicitly blocked (which is not the case here).
It is completely normal that connections to some nodes get dropped or that some nodes do get banned from connecting to your own node.
This is because the nodes are not being compliant to the network rules.
Checking the firewall is necessary if ALL connections (or the majority) somehow get rejected / dropped. Not if you just can't connect to one single node while still having enough incoming/outgoing connections.