It's from pasting in the forum. In my key file are no newlines.
After some contemplation of this issue I found the following article
Might be that I have to convert it somehow to a standard ssh format in order to log into the machine.
ssh-keygen -e -f path/to/opensshprivate.key > path/to/ssh2privatekey/ssh2privatekey
So I tried to use the formula mentioned in the article to no avail.
ssh-keygen -e-f /home/user/id_rsa > /home/user/standardsshkey.txt
Load key "/home/user/id_rsa": invalid format
Guess the Meta machine at HTB keeps the user flag heavily guarded...