So I was able to get my project to build in UE 5.5 by disabling Unreal Build Accelerator, which I did from the engine’s buildconfiguration.xml, as outlined in this tutorial: Disable UbaSessionServer (UBA) | Community tutorial
After disabling it I was still getting cook errors, so I:
- Deleted the Intermediate and Binaries folders
- Regenerated the visual studio project files
- Built the project within visual studio
- Then tried packaging again and it finished successfully
Not sure if all of these steps are necessary. In fact after it packaged successfully I ran it again with Unreal Build Accelerator enabled again, and it produced the Error 9666 again, but those build steps were automatically retried with UBA disabled, and the packaging still completed successfully.
I tried looking through the source code but since I couldn’t dig into the ubahost.dll file to see what was producing Error 9666, I wasn’t able to determine what the specific issue was.