I want to use the Yubikey to generate my Master Key.
Question: How can I use ore open the Keepass-Database when I lost the Yubikey? Is there a fallback solution to open the database?
I assume you are using the KeePass OtpKeyProv plugin to unlock a KeePass database with a YubiKey. The OtpKeyProv plugin uses a shared secret (recovery) key with the OTP generator (Yubikey). This shared key (not the OTPs) is the actual key file portion of the Master Key. As long as you retain the secret (recovery) key you will be able to unlock your yubikey protected database using the secret key as the Key File portion of the Master Key instead of an OTP.
thanks for the tip. I will test this.