UEFN Versioning Stage Failed: Fix Launch Session and ValkyrieUploadTemp

UEFN versioning stage failed after you pressed Launch Session? This error appeared with Fortnite’s v42.40 update on October 15, 2026. Preparation, Validation and Upload can all complete successfully in Session Inspector before Versioning turns red and stops the entire launch.

Epic marked the original problem resolved on October 22. Before trying an old workaround, check for a newer UEFN update. If the same failure continues, Epic’s suggested fix was to clear one specific temporary upload folder and restart the editor, without touching your island’s source files.

UEFN versioning stage failed: what the error means

A successful session prepares, validates and uploads the project, then creates a version before Fortnite cooks, downloads and launches it. In the reported bug, the first three stages completed, but Versioning failed. The rest of the steps were cancelled, leaving no session to join for playtesting.

Epic tracked the issue as FORT-1173189. Several creators seeing UEFN versioning stage failed confirmed the temporary-folder workaround helped after a complete UEFN restart. That doesn’t mean the same fix works for every editor or asset problem.

Check your UEFN version before clearing anything

Epic marked the original v42.40 problem resolved on October 22, 2026. First, close UEFN, check Epic Games Launcher for an editor update and install it. That could save you from troubleshooting a problem already addressed in the software.

If UEFN versioning stage failed still appears, open Session Inspector and check which step turns red. This workaround matches the case where Preparation, Validation and Upload succeed but Versioning fails. An earlier error needs a different diagnosis.

How to remove the ValkyrieUploadTemp folder safely

Epic’s engineer advised Windows users with this exact problem to delete only the temporary ValkyrieUploadTemp folder and restart UEFN. This directory holds temporary upload data, not your island’s source files. The shorter File Explorer path is:

%LOCALAPPDATA%\UnrealEditorFortnite\Intermediate\ValkyrieUploadTemp

The full Windows location is C:\Users\%USERNAME%\AppData\Local\UnrealEditorFortnite\Intermediate\ValkyrieUploadTemp. File Explorer accepts the shorter %LOCALAPPDATA% path above, which takes you to the matching folder for your Windows account.

• Close UEFN completely. Make sure no editor session is still running.
• Open Windows File Explorer, paste the exact folder path above into its address bar and press Enter.
• Confirm the directory is named ValkyrieUploadTemp and that you’re inside UnrealEditorFortnite\Intermediate.
• Delete only ValkyrieUploadTemp. If the folder does not exist, don’t delete an alternative directory.
• Open UEFN again, load the affected island and try Launch Session or the upload once more.

The UEFN versioning stage failed workaround is limited to that scratch folder. Do not delete your Fortnite Projects directory, Content, Verse code, the whole Intermediate directory or the entire UnrealEditorFortnite folder. If ValkyrieUploadTemp isn’t there, don’t choose another folder to delete.

What if Versioning still fails after the restart?

If UEFN versioning stage failed returns after the restart, try launching another project. If one island fails but another launches normally, the problem may be tied to the affected project rather than the editor’s session service.

Read the Session Inspector output and UEFN logs for the exact failure. Some October reports mentioned MODULE VALIDATION FAILED, CandidateApiJsonInvalid and Differing redirectors. Those details help when reporting a problem, but they don’t establish the same cause in every project.

Epic also suggested the cache workaround for some v42.40 validation problems involving redirectors. It will not repair a genuinely broken Verse script, missing asset dependency or every other publishing error. Avoid deleting more files just because the first attempt didn’t help.

Where to report a UEFN versioning error

If it still fails, gather your UEFN version, the failing Session Inspector stage, the exact log messages and whether another project launches. Include those details in a report through Epic’s official developer support channels, rather than chasing guesses from unrelated error threads.

The original UEFN versioning stage failed reports began on October 15 and were followed by Epic’s October 22 resolution notice. This is a guide to that specific failure and its documented workaround, not a claim that all UEFN sessions are currently broken.

Does this affect publishing your island?

Launch Session is for testing an island, while publishing it through Creator Portal uses separate checks. If missing thumbnails or a transparent island logo are stopping a release, our Fortnite island thumbnail requirements guide covers those files.

If UEFN versioning stage failed still appears, update the editor, confirm Versioning is the step that fails and use only Epic’s specified temporary-folder fix. If the island is already published but the Library tab isn’t announcing updates, that’s a different issue covered in our Fortnite Discover notifications guide.

Official source: Epic Developer Community: UEFN Session Launch Fails at the Versioning Stage.