We still didn't find the cause of the problem, but the following helped some people:

Uninstall Divinity (make a backup of your savegames)
Install Divinity again
Apply the latest patch (DO NOT APPLY THE PREVIOUS HOTFIXES)
Load the savegame

We are looking further into the problem and we will release a fix the moment we find out how this can happen.

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Little remark: "I made backups from my savegames and now I can’t load them anymore" --> When you copy the savegames from a CD to your HDD, the read-only flag is set by Windows, preventing Divine Divinity to open these files for writing. Clear the read-only flag by right clicking on the files, selecting properties, unchecking the read only flag and then clicking on apply.