When I try to checkout a file, the following message appears as it is supposed to:

      

But soon the Checkout icon next to the filename changes to a padlock icon, in 5 seconds or so:

The file opens, but this message appears on top:

If I close and reopens the file, the following message appears:

     
    (Yes, it was me who checked-out the file!)

I am able to work offline, but the Checkin and Checkout commands become dimmed and Cancel Checkout command does not work.

The server's administrator says I did the checkout correctly and that all is ok there, that is, permissions are OK and the server is available.

Does someone have any clue?

(PS1: I'm working with a trial of SDL Trados Freelance Edition, so I do not have access to the SDL support)
(PS2: The problem occurs also in SDL Trados Pro 2019, 2017 and 2015, unlicensed)

Thank you all!