@bentael Thanks for getting back to me and for all your help. In the end, Nifty took over my access to DigitalOcean and managed to set up Nodebb with my Mybb import.
It didn't work out the way he wanted, he had to dump on my droplet, the install he had worked out on his own computer with the import already included.
His guess was that my droplet was fighting the import since I had chosen the 512mo ram plan.
You know that if you edit a file of NodeBB itself it may refuse to update by git pull because a merge would be needed?
So it may be that you're working with an old version of NodeBB o_O
If you are able to delete a post you have the permission to also restore it. Because of this the post doesn't get removed but only gets less opacity (within the official themes).
I just don't know in which case the post could be removed from DOM after reload, may be an inofficial theme or an old version of NodeBB (maybe because of the reason above).