@Vincent: The region setting is the same (unchecked) on both machines. The MSSQL Client driver is the same version as well.

@Wil: It happens in cWebEdits, cWebForm and in cWebColumns (I did not try any other controls).

I'm going to delete my SQL Database and "start from scratch" (eg. load a working backup and do the migrations to Unicode again on my machine). Let's see if it is something I accidentally changed in the database settings.
If that does not work: I'll restore a backup from the other machine locally and see if that helps.

As it seems to work on the other development machine, I assume it's something wrong on my machine. I tested accessing WebApps hosted on other machines and they work fine. I assume something is wrong with my installation.