SDL WorldServer - Error when importing/uploading Translation Kit as ZIP file after XLIFF (not WSXZ/sdlxliff) export: 'Import Offline Work' 'No items were imported' and Value cannot be null. Parameter name: SDLWS:DocId not set!

A project's Task has been exported in XLIFF format. After translation, a ZIP file has been created and the transaltor attempts to import it back into the SDL WorldServer project. However, the import fails with error:

Value cannot be null. Parameter name: SDLWS:DocId not set!

Important:  we do not recommend using the XLIFF export format for translation. XLIFF export format is not the same as *.sdlxliff. This format is not totally compatible with a live-TM WorldServer system and should not be used. The export format should be the Studio package WSXZ format (which contains files in bilingual studi format *.sdlxliff). The WSXZ package should be opened in Trados Studio as a package, translated, and then a Return package should be created and uploaded.

If you "must" use the XLIFF format, the right process must be applied, which is:

1- Open the XLF file exported from WorldServer in Studio. A SDLXLIFF file is created.
2- Translate the SDLXLIFF file in Studio
3- When you are done with the translation, go to File/Save target as and create a target XLF file. This is the bilingual file that includes the translation and should be imported/uploaded to WorldServer.
4- Finally, zip that file and import it to the WorldServer project.

To fix the issue in an existing ZIP file that contains the SDLXLIFF file and hence cannot be imported/causes the error, follow these steps:

1- Unzip the ZIP file. It will contain one or more SDLXLIFF files
2- Open each of the SDLXLIFF files included in the ZIP file in Studio. The SDLXLIFF files are expected to be fully translated.
3- Create a target file (go to File/Save target as... and save the target file to your local computer). A target XLIFF file will be created
4- Now compress/ZIP that target XLF file
5- Upload it/import it to WorldServer.

These articles might be relevant:

WorldServer - Error or unspecific message uploading Return package to WorldServer: 'Import Offline Work' 'No items were imported' or 'DocId not set'
WorldServer - Error 'Placeholder not found' when importing a XLIFF file