The message "The 'FEIID' attribute is invalid - The value 'xxx' is invalid according to its datatype 'http://www.fei.org/Result:stHorseFEIID' - The Pattern constraint failed." appears if the FEI ID of the concerning horse is not in the expected format.

Most likely is that horse horse has been imported from an external source and the FEI ID is not checked.

Search the correct FEI ID in the FEI Database (https://data.fei.org/Horse/Search.aspx)