Hi, thanks for the fast answers.
I thought that plugin was the one causing problems as when it was deactivated UpdraftPlus worked perfectly. I'm currently using Chrome's last version and I tried with Firefox as well and it would throw out that error until I removed the seo plugin. These 2 are not, however, the only plugins installed.
I'm using a custom theme so it might be something related to that as well. I'll try to debug it whenever I get the time and see what's causing the problem and I'll try to keep you updated about this.