We ran into the same problem. You may find a solution here [OJS 3.3.0.3] Errors when trying to export issues
I assume, this error occurs, when a “file stage” was removed in a newer OJS version. The file stage is saved as a number at every record. There might be old entries with a file stage, which does not exist anymore in the corresponding classes file pkp-lib/SubmissionFile.inc.php at 6375f0ac97ea4a93381ce8851a6c871a32415583 · pkp/pkp-lib · GitHub after an update of OJS. Therefore the “stage” is now empty, which produces this very XML error.