Hi Jordi,
I tried the same steps as before, but this time using OMERO server 5.4.7 and was not able to reproduce what you have seen.
In the meantime, what may fix the issue for you is to delete the thumbnail folder in the omero data directory on the server (the directory is configured on the server as omero.data.dir and there is a Thumbnails folder in there). The thumbnails should regenerate when you try to view the images again and hopefully they should appear in OMERO web.
Thanks,
Riad