The smaller window is for master password.
The larger window is for your password of your account on the remote server.
I'm not sure which you mean by "main" password.
So did you enable encryption of stored passwords using the master password by purpose? Or did you do it by mistake?
https://winscp.net/eng/docs/master_password

Yes it is.
Everytime: at first 2 times a small login field, both giving an error message. Then a larger login will appear. I can enter the master password there.

Is the master password enabled in preferences?
https://winscp.net/eng/docs/ui_pref_security

I manage several websites and log in with WinSCP.
Yesterday without any problems, and now suddenly a master password is requested.
Updating did not solve the problem.
How to solve this problem?