Timeline rendering fine in various settings, but ***not*** uploading on any platforms

Hi everyone,
Let me start by apologizing for asking a question right away without making a presentation topic first. I didn't found the presentation section and my question is pretty urgent, so please forgive me if I'm skipping a step here!

Anyway, here's my problem : Using FCPX 10.4 on Mac OS Mojave 10.14.2, timeline settings 1080p 30fps (native 1080p not proxy), I rendered a promo video for a client in many different ways and tried uploading it on many different platforms without success.
Render and playback on my device is going smoothly. Uploading directly to YT to share with my client works fine on my end, but my client cannot see the video. I did it twice in a row. I can see it just fine.

I have tried uploading an H.264 proxy file for feedback to Sync.com *and* Dropbox : Both platforms flat-out refuse to upload the file.
I've rendered it again a couple of times, still, the same thing happens. I switched internet connection, tried uploading on FB as well (you really have to be desperate, right?!)... nothing works.

Again, timeline render is fine on my own, but somehow, the files I export cannot be uploaded. I heard there is a possibility that FCPX might spoil the files if you don't update regularly. Is that just internet gossip?

Because the version I have installed on my work laptop comes from my employer and is registered under their legal name.
Do you guys think I need to ask for an update then?

Any help would be greatly appreciated!
JB

If you upload to YT and you can play it fine, and your client can't, that is NOT an FCPX problem, that is a YT problem.

Be aware that YT and other online services take time to prepare/convert your video before it can be seen by others. But if you can see it, they should be able to see it. And if this happens on more than one platform, your client has an issue on their computer.

I upload files daily for my TV station, we export as either Video+Audio H.264 or ProRes 422 and have NEVER had an upload issue that was FCPX's fault.

As for updates, they are free, be sure your OS and apps are all up to date. But that shouldn't matter for exporting a file.

Hi, thanks for your reply!
Seems like my problem is solved, but I'm not 100% sure how... anyway... Just to be clearer, there were multiple issues :
Some files straight out refused to upload from my computer, and I had error messages on *my* computer, on various platforms, *before* the file was uploaded at all (even tho FCPX actually rendered them fine as far as the eye can see, so it could be metadatas).

The two files I uploaded to YT uploaded fine to *me*, but not to my client.

I then did a test with a colleague through WeTransfer, and everything was fine.

The only thing I did differently, is that I selected the timeline before going into share => masterfile => H.264

I dunno if that changed anything - I normally don't specially select anything and it renders it the same.
Might be that?

As for the updates, as I said, the app does not belongs to me, it belongs to my employer, so it is not linked to my account, and I therefore am not responsible for updating FCPX on my computer (sigh...).

Thanks for your help
I hope to have it figured out, but probably it was a mysterious glitch which origin will remain unknown.