Guest User cannot upload File in Form - An unexpected error occurred

Issue

We have a Form with an Upload field where authenticated users can upload files but Guests cannot.
All permissions are provided for Guest user.
When the user selects the file to upload, an error message appears immediately: "An unexpected error occurred while uploading your file."
If we add "com.liferay.dynamic.data.mapping.form.web.internal.upload.DDMFormUploadResponseHandler" with DEBUG in Server Administration > Logs, we see the following error:

DEBUG [http-nio-8080-exec-630][DDMFormUploadResponseHandler:56] null
com.liferay.portal.kernel.exception.NoSuchUserException: No User exists with the key {companyId=20101, screenName=ddm_form_default_user_screen_name}
    at com.liferay.portal.service.persistence.impl.UserPersistenceImpl.findByC_SN(UserPersistenceImpl.java:4582) ~[portal-impl.jar:?]

Environment

Liferay DXP 7.3 under Update 31
Liferay DXP 7.4 under Update 76

Resolution

When "Autogenerate User Screen Names" is ticked in Instance Settings > Users > Fields, the ddm form default user is created with a screen name of "ddm.form.default.user.screen.name".
This differs from the screen name the DocumentLibraryDDMFormFieldTemplateContextContributor expects ("ddm_form_default_user_screen_name"), so any form using an upload field throws an exception.
A fix was implemented with LPS-133096 from u76 for 7.4 and u31 for 7.3.

Additional Information

The following workaround is available:

Navigate to Control Panel > Server Administration > Script
Run the following script:

import com.liferay.portal.kernel.service.UserLocalServiceUtil;
import com.liferay.dynamic.data.mapping.constants.DDMFormConstants;

UserLocalServiceUtil.updateScreenName(userId, DDMFormConstants.DDM_FORM_DEFAULT_USER_SCREEN_NAME);

Before running the script, make sure that you create a backup from the database so that it can be restored if needed.