What to do when the Mail app shows blank "From" section on my iPhone?

Hi everyone,

I am having trouble with the iPhone mail app. Recently, I created a new primary iCloud email address, and it has been working great on my Macbook aswell as on iCloud.com. However, whenever I try to send a mail from my iPhone, the “From” section is just blank, meaning that I have to tap the section and pick the iCloud address (which is the only one showing up) everytime.

My feeling is that the problem is caused by the standard-e-mail being @me.com. The handle of the @me.com address is equal to an old gmail-account that I used 10+ years ago to create my Apple-ID with. I am not able to send anything from the address, but I cannot change the standard-e-mail from being the @me.com address to my new iCloud e-mail.

Please help!

[Re-Titled by Moderator]

Original Title: Problem with iPhone mail app