I can't login into my Sync Account. I get the authentication Code but I'm stuck in a loop.

Hi. I try to be as clear as possible. I have 2 email accounts: Johannes2922@outlook.de and regeneration35@Mailfence.com. regeneration25 is the primary e-mail from my synch account. I don't know if I should give this information here and therefore I changed the numbers a bit but the problem lies between these 2 emails. I also deleted another Firefox Synch account which had regeneration35 as primary e-mail. So I can use this e-mail as my primary e-mail for my current account. This might be little details that could be useful for the problem. If I try to sign up with regeneration into my synch account, I get incorrect password error and can't login at first but then I click on "use different account" and it looks like on picture 1 I put in images. I type regeneration35@Mailfence.com again into the text field and use the same password to get the authentication e-mail with the code. So now I get 2 authentication e-mails on my regeneration35@Mailfence.com and Johannes2922@outlook.de, 1 for each. The authentication code used from my primary e-mail doesn't work. Here lies the problem. It brings me back to the login screen window as you see in picture 2. What can I do? If I try to use the authentication code sent to my secondary e-mail I get the same result. Then I'm stuck in a loop as I wrote on the Subject.