First of all thanks guys for your fast replay.
To agsteele
YOU: "You could try uninstalling and reinstalling".
RE: already done, unfortunately nothing has changed.
YOU: "It is possible that you have mistakenly used an incorrect username/password pair".
RE: nope.
YOU: "I would check the number of devices that Evernote thinks you have once more".
RE: I did it and, before reinstalling Evernote Legacy, the tablet showed to me that at that moment I had only associated 1 device. That is the tablet itself. To tell the truth, I doubted that the Evernote app installed on the tablet was providing me with incorrect information. But I couldn't/wasn't able to control this.
To Dave-in-Decatur
YOU: "you'll probably need to contact Evernote support".
RE: I already did. Despite the free account, someone from Evernote answered me. But they told me to install the latest version and give it a try. The problem is that, for various reasons, I don't like the latest version and, what's more, it eats up a good chunk of RAM in my old laptop...
Any other suggestion? Thanks anyway for your kindness and time.
Cheers,
Andrea