I can only use a block on a carpenters block or stair or whatever when i'm in cheatmode. This is a little bit annoing because i like to play without cheats. Am I the only one who expieriences this bugthing?

Probably ownership problem. I suggest first to try disabling ownership in the config file.

I changed default ownership to player name which will fix this type of issue in singleplayer. It was just a side-effect of resolving issues on offline-mode servers.