As per checking, there are no current cases with projects. With this, let's perform a browser troubleshooting, @ELM tree.
However, before we begin, may I ask if your projects are missing when running a Report or if the project is missing in the Projects tab? Any additional information will be highly appreciated.
In the meantime, let's perform a browser troubleshooting to determine if this error is browser-related. QuickBooks encounters errors like this due to accumulated cache.
To begin the process, you can open a private browser, as this ignores historical data. To save you time, use either of these keyboard shortcuts: