Sharing error "background tasks that are not yet complete." Sharing is idle!!

This is a bug. I first hit it right after updating to 10.3.4. For me it is not on stabilization but the new Flow transition. The problem does not happen on all Events or Projects, just seemingly random Projects. I can open another project and Share just fine. The error even happens when Sharing Current Frame unrelated to any clip with a Flow transition. The error is preceded by a progress indicator that FCP X is preparing for a render, that takes 20 seconds and then the popup. So you can't Share and walk away which is annoying. When it works, Share Current Frame only throbs for 5 seconds.

In my most recent case, opening the compound clip allows me to successfully Share Current Frame. But the same frame cannot be shared from the "master" timeline where the compound clip is used.

I've tried erasing all Event and Project files to no avail. I've also tried the (suggested by some) remedy of using the down cursor to visit every seam in the project, let the render throbber throb, then move to the next. I have also replaced every single Flow transition. None of this resolves the issue.

In my case the Flow transition renders fine in the Share but whose to say it will be in every case with a bug like this? YMMV

EPILOG: Yup this is a bug. I opened the compound clip I was trying to Share from and sharing the same frame from inside the compound clip worked fine. Back to the compound clip and "no joy". I reported my findings. YMMV

This has been going on for a long time since before 10.3. I always ignored those types of messages like the one that says something along the lines of "something is missing" and never had a problem with the final output. It is a bug I have reported several times. I haven't seen it so much lately though. Also, I sometimes get the yellow caution sign when there is nothing missing in a project. Exports are fine. Again - it's been reported.

I'm having the same problem. I've tried exporting an xml of the project and importing into a new library. I also tried deleting all the rendered cache files, but no luck.

Since the message specifically refers to stabilization, maybe you should try turning it off.

Can you then export?

Then you may turn it on again and see if it completes successfully.