Sorry for the delay, the passphrase is 512 characters long (now I should change
it after publishing that here ;-)) and just ascii characters.

Hello, Thank you for your reply.

I used the gpg4win-2.2.1.exe binary which I downloaded from gpg4win.org

The popup I mentioned is the screen that asks me for my password when I try to
open an encrypted mail in my mailbox via thunderbird/enigmail. See the
screenshot. In the newer gpg version this popup is replaced by a prompt screen
that says pinentry and will allow only for shorter passwords.

I understand that my password is exceptional long, as I still was (and maybe
still am) a beginner on the encrypted mail part. But backwards compatibility
seems pretty important in the case of encrypted mails and passwords to decrypt them.