After months of developing a new company product with Eclipse using the local Notes client to access databases etc. I started to create another customer app today. I also did this some hours. All of a sudden, Eclipse prompted me to enter my Notes-ID password when I wanted to test my code. After ignoring it (as it won't work anyway) I wasn't able to access a database on my server anymore. WTF?
Long story short: in between sessions I switched my user ID to the customer's ID as I had to access his server. Though I changed back to my local DEV ID the problem popped up.
This proves that policies work as a charm. It seems that the customer uses a policy to set (forced) the security option
"Don't prompt for a password..."

in the user's security settings of the client.
Again, Jesse Gallagher was helpful and reminded me of this setting. After enabling it again, everything worked like it did months before.