So, last night (while chatting to Bob ;) ) I managed to find the problem of my disappearing content. I was a messup in the database in the DataTypes. Read more about it here.
On the other hand, I feel also pretty stupid. Why? Because only friday-night I found out that there's a nice feature in Umbraco: Rollback.........I could have spared much time by just using the rollback-function to get my disappearing content back......
Oh well, knowing that my issues seem fixed, I can get back in business now!
Thanks everyone that tried to help fix the problem!