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Lars,
this will probably not be the cause, but to be sure...
could you make sure you uploaded the following files:
in the /include directory: common.php
in the /class directory: xoopsblock.php, template.php
and how big are those files? I mean they should be bigger than 0kb
Because there could have gone something wrong while uploading the files...
Otherwise it might be something with the configuration of xoops. You could check the settings in mainfile.php I believe. But if something is wrong in mainfile.php you'll get a lot more warnings.
Good luck!