@LennardF OK, I think we need to sort out what you actually want to do:
The headline of this topic is “Unable to load PBEM map”.
The first screenshot you have posted shows an error to post to the forum, that is PBF, and unrelated to PBEM.
Your last posting indicates that you want to play by email, what means sending the savegames by mail, instead of posting to the forum.
Now it looks lke you can load the savegame/map without issue (different from the headline), but there seems to be a misunderstanding in how to move on from there.
This is how the screen looks when you have selected your savegame:
Now if you wanted to play by forum you would need to hit “Play by Forum” and you will get this:
Here you would need to add your forum-login name (LennardF) and your forum password.
But this is only required in case you want to post to the forum.
Now if you wanted to play by Email instead you would hit “Play By Email” accordingly and you will get this:
Here you need to enter the settings needed for your email-account. This is totally independent from and unrelated to any forum settings. TripleA acts like an Email software in this case.