Hey,
That sounds like tmp-plugin is unable to crate a connection to our servers.
There are a few possible reasons why that would be happening.
One of them being that our servers would be down. However, even tough this wouldn't be completely unlikely scenario, based on our around the globe monitoring it appears that at least for past few days there has been absolutely no interruptions in our service.
Thus, I would guess that it would be something on your host or in the internet connection between your computer - isp - the internet.
Since you were able to send this email, I'd say you still have a working internet connection. Thus we could rule out general issues with your internet. (Though it's still possible that your ISP hates us and are blocking only connections to us...)
After this wall of text, a few more realistic scenarios and how to test them.
So to tighten the scope, a few questions
1) With your favourite internet browser, can you access our sites; especially the tmp-server - same url as the one in tmc-settings in netbeans - most likely
https://tmc.mooc.fi/mooc
If you can access it with your browser and TMC-Plugin in NetBeans is still giving you this error message, it if probably either your anti-virus, firewall or maybe you are using a proxy to connect to internet.
If you are using a proxy, check that NetBeans is configured you use the proxy too.
Use the (4) Test Connection first to check whether NetBeans thinks it has internet connection, and if/when not, thy reloading the proxy settings with (3)
Let me know if some of these steps/ thoughts helps or if these didn't help you to get the connection working.
Cheers,
Jarmo