Is this the same error as you showed in this previous thread? If so, did you follow the advice given there? With what result? (We must assume that, given that thread is from 2020, the problem was solved!)
I think that message suggests a server error, i.e., something at the Magix "end" and so nothing that is within your power to change.
You could email infoservice@magix.net and ask whether they are aware of this problem and, if so, when it is likely to be resolved. If you do want to do this I would recommend sending the email as soon as possible if you would like to receive a quick answer, since the customer service "desk" is in Germany where the clock is 1 hour ahead of UK time. (Apologies if you already knew this.)
I can connect to Magix Online with Web Designer, no error messages, publishing a site goes through the process, and using FileZilla I can see that the web files have been updated, however it did take a while for the browser to get the updated version.
Is your subscription up to date for Magix Online?
Can you connect with an FTP program such as FileZilla