possibly reasons:
antivirus software
task offloading
faulty router
wrong/old network driver (esp. for a virtual machine)
bitcoin.conf configured to only connect to one host, that's not taking connections anymore
I don't think it is the router because other wallets are working fine. Also not a driver issue because the problem is on two seperate computers.
There is no bitcoin.conf file
Antivirus/firewall and all security software is off.
What is task offloading?
Also, if I call my ISP, what am I suppose to explain? What port does bitcoin use? Maybe they have closed it or something...
I just edited post to include one that's running on non standard port... that'd probably be the first thing to try
if you're on windows, you can either use netsh from the command prompt or use the control panel -> device manager -> network -> properties, then it should have offloading options there, amongst other things. for linux you can use ethtool,
https://www.kernel.org/pub/software/network/ethtool/ethtool-3.15.tar.gz .. compile & install then ethtool -k eth0 (or w/e) will show options, ethtool -K eth0 <xx> to change..
i wouldnt mess with those until checking everything else though.
the whole other side closing connection is typically what would happen when a server that is already at its max number of connections is being connected to, afaik (it closes immediately after opening)
try going into console and doing addnode 5.9.24.81 onetry also, if the other one doesnt work (or in linux bitcoin-cli addnode xxx onetry)