I disabled also the theming app, and nothing changed.
I found this:
which led me to discover that the problem is the same: https://mynext.cloud/index.php/css/icons/icons-vars.css cannot be loaded.
The file is of course present under my data dir, where it should be.
The problem should have been fixed with the fix proposed here
but doesn’t seem to work, the problem should be somewhere else…
It seems I solved my issue.
My NC is hosted on OVH shared hosting, and I had PHP instead of PHPCGI in the configuration.
Switching back to PHPCGI instantly solved the issue.
Hope this is useful for others too!