Nodes with good inteet connection can't communicate between each other

I have a server which suddenly wasn't available anymore. After long investigation I've found out the following:

• when I ping it from my local network ~20-25% of the ping packets is dropped
• I can't really access any http services running there
• ssh is failing constantly, mosh used to work, is failing now as well

but:

• my home computer currently has 40MBit down and 9MBit up (measured via speedtest.net)
• the server in question actually has >90MBit down and >80Mbit up (measured via command line version of speedtest.net)
• I can connect to a 2nd server just fine and access all websites via curl just fine
• curl from my local computer times out
• DNS resolves the correct IP

What am I missing here? - and yes, no computer in my local network can connect to the server in any decent way.

Uptimerobot is also reporting that everything is up and running while on my side the network communication is down for the last 2 hours. I'm pretty sure by tomorrow it will be resloved but I'd like to know for the future what other things I can do to resolve it.

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