The database expects the value in the “vk-Nummer” column to be a valid GUID, but it is apparently encountering a blank, malformed, or incorrectly mapped value. A GUID normally looks like 6F9619FF-8B86-D011-B42D-00C04FC964FF. Microsoft confirms that an incorrect length, unsupported characters, or an unrecognized format causes GUID parsing to fail. Therefore, repairing or reinstalling Excel is unlikely to resolve it.
Could you please provide the name and version of the database application from which you are exporting? Does this happen with every list or only one particular list or record, and does Excel open before the error appears? If possible, test a different or smaller list and try exporting as CSV.