You know, there is a part of the forum wher similar issues are handeled, topic moved there.
The error is indeed a connection problem.
The first thing for you to check now: are you able to login on the player page?
https://www.maniaplanet.com/login
Then, check Windows Defender, your firewalls and your antivirus, to make sure that Maniaplanet.exe is not blocked.
If you don't know how to check that, please uninstall Maniaplanet.
Then install it again, using administrator rights.
And also start Maniaplanet with administrator rights the first time.
There should be a popup telling that Maniaplanet would now test the network connection. Eventually your security suite will show a message to allow Maniaplanet, and this is where you have to accept.
Also, do not try to use TMU and Maniaplanet at the same time. This would also explain why you cannot connect, as the ports used are similar if not the same.