Import failed Please select a database file to import

I exported the json file from the production site, imported it into the local Ghost installation and did some post editing and insertion.

If I export and import the contents from settings> labs locally this works without problems.

If I try to import the same json file on the production installation of Ghost I get the following error:
Import failed Please select a database file to import.

Ubuntu 18.04
Ghost 5.14.1 (both local and production)

importing this json works on a newly installed machine with Ubuntu 20.04 and Ubuntu 18.04

what can be the problem with my production server?

any suggestions? it is quite urgent, I can’t find a solution at the moment

I found this post on forum, I’m not using the API directly, but it could be a similar problem

is there no way to fix it? I couldn’t do it by myself. I have to reinstall all the machine?