So I changed the console login password, and the first message disappeared, and I managed to re-upload the file. So for those who might have this problem, it's because your password contains special characters like é, è, etc., so be careful.

Maybe that's it...

I changed the admin password to simple letters and it works, so it's the character conversion that's messed up. How can I fix this, please?

I also got another message after that:

[Window Title]
waptconsole

[Content]
Error loading agent...

Apparently, it's a problem with Python.

Jul 3 16:12:10 wapt python[6259]: [2018-07-03 16:12:10,851] ERROR in app: Exception on /api/v3/upload_packages [POST]
Jul 3 16:12:10 wapt python[6259]: Traceback (most recent call last):
Jul 3 16:12:10 wapt python[6259]: File "/opt/wapt/lib/python2.7/site ...

Hello,

I installed WAPT on Debian as instructed on your website. No problem there;
I installed the console on the PC and created the certificate.
When I create the agent, it correctly asks for the passphrase, and then immediately after, it displays this message:

[Window Title]
waptconsole

[Content]
Creating...